There are many more powerful PDF and image editors, yet plenty of us use Preview for those basic little tasks, where we don’t need the complications of a heavyweight. Preview isn’t without its faults, though. It can mutilate PDF annotations made by other apps, and sometimes unexpected things happen. To deal with those it has one feature that’s almost unique among PDF and image editors: it automatically saves versions using the macOS versioning system. This article explains how to make use of that.

Ask to save changes when closing documents

One important control over the behaviour of Preview and many other apps that you may not be aware of is this in the Windows section of Desktop & Dock settings. When combined with apps that use the macOS versioning system its effects are significant.

Ignoring versioning for the moment, when this setting is turned on, if you go to close an open document that has unsaved changes, before it’s closed you’ll be asked whether you want to save that changed document. This is a long-standing safety net that continues to protect us from losing lots of work by accident.

When this setting is turned off, apps may still ask you whether you want to save unsaved changes before closing a document unless the app uses the macOS versioning system. In that case, the app automatically saves a new version without offering any option. This might appear inappropriate, but as that’s non-destructive, it avoids interrupting your workflow: you can always revert to the previous version of that document, provided that you’re aware that it has been saved automatically.

If you’re not aware of what’s going on here, and how the versioning system works, this can cause odd effects you can demonstrate using Preview.

Preview’s saving grace

To see how Preview handles versions and the effect of that setting, find a copy of a suitable PDF or image file and duplicate it in the Finder. Name one copy something like testSavesOff, and the other testSavesOn. Set Ask to save changes when closing documents off to begin with, and open testSavesOff in Preview.

Now perform some destructive editing on that document without saving it, here a radical crop.

Leave it a couple of minutes before closing the document. Following that setting, Preview shouldn’t ask you whether to save the changed document, but will simply close it. Then quit Preview.

Now set Ask to save changes when closing documents on, and repeat the same sequence with testSavesOn instead. When you try to close that document, Preview should now ask you whether you want to save the changed document, to which you should click on Revert Changes to set the document back to its previous state instead.

Inspect the two documents using Quick Look and you’ll see that testSavesOn hasn’t changed, but testSavesOff has, although in neither case did you save those changes yourself.

All in the versions

Although you can see what has happened using Preview’s Revert To menu command, the clearest way to see what has happened to those two documents is to open them using my free Revisionist, which shows their saved versions.

testSavesOn has three saved versions. The current one is the same as the original, but the second version shows the destructive edit that you didn’t save.

testSavesOff has two saved versions, the original, and the current version is that after the destructive edit, which you also didn’t save.

Thanks to Preview’s use of the macOS versioning system, either way you’ve still got access to both versions of that document.

One point to note, though, is that the versioning system doesn’t automatically clean up old versions for you. When you’ve finished editing a document in Preview and don’t want to retain its old versions, delete them either using Revert To in Preview, or with Revisionist. Versions are only retained for the original document as long as it’s stored on the same volume. So you can also wipe old versions by duplicating a document and trashing the original, or copying it to another volume. Once they’re gone, you can’t restore them, as not even Time Machine can back up versions.

In addition to reading laid-out documents, the most popular purposes for PDFs are forms and annotation. As far as filling in PDF forms are concerned, I have just one word to say: Fillably, Joel Norvell’s outstanding app available from the App Store, which transforms Preview into the ideal platform for tax and other forms. Rather than struggling with the tangle of tools in a general PDF editor, Fillably provides the perfect suite for creating PDF forms.

Annotating PDFs is more complicated, though.

Encoding annotations

Annotations aren’t an afterthought, but a central part of PDF. All PDF documents consist of a list of hundreds or thousands of objects of different types, including annotations of the Annot type, and those are listed in one of the file’s standard dictionaries, its Annotations Dictionary, Annots.

There are at least 27 sub-types of Annot, including Caret, Highlight and Stamp, which are reflected in the annotation tools provided by apps from Acrobat to Preview. Seemingly complex annotations like popup notes are straightforward to code in PDF, requiring just two linked objects, one for the popup and its text, the other to specify its placement on the page. Others are more involved, as they can extend to include file attachments, sound, movie and other multimedia.

PDF versions

Despite their original simplicity, there are multiple problems that can arise with annotations.

With more recent versions of PDF, the ways in which they can be coded has increased. Mark up a PDF using the latest versions of Adobe Acrobat Reader or its ‘Pro’ CC colleague and they’ll cast it in PDF-1.6 and you’re unlikely to see a single Annot in their source. Most apps built on the Quartz PDF engine should write their files in PDF-1.3 so they can be accessed more widely, and should use regular Annot sub-types throughout. However, Preview likes to use opaque AAPL:AKAnnotationObjects that you won’t encounter anywhere else.

What Quartz does is to ‘flatten’ each PDF into a common 1.3 format for rendering, and that can be saved to disk. At present, that seems to work faithfully, but might give the impression that macOS can’t render more recent versions of PDF, which isn’t true. You can demonstrate that by opening an Acrobat PDF-1.6 document using an app that relies on the Quartz engine, such as PDF Expert, Preview or my Podofyllin, and comparing that with the original in Acrobat.

Podofyllin has a convenient feature for doing just that, in its source window. The uppermost of its three views displays Quartz ‘flattened’ code in PDF-1.3, the middle shows the original, here in PDF-1.6, and the lowermost is a summary of the latter.

Hidden annotations

The biggest dangers with annotations arise because of PDF’s ancient origins and a file format that doesn’t make sufficiently clear distinction between data and metadata. All annotations are metadata added to the underlying document, but in PDF, objects for each are mixed freely within the source. When they’re clearly distinguished with the Annot type, they should be easy to remove as a group, and PDF Expert offers that as a convenient command. That’s ideal when a document has been developed with the aid of reviewers’ annotations, to prepare the finished version for release.

Unfortunately this can cause its own problems, as PDF source is notorious for retaining old content that’s no longer visible in the rendered document, but can be read by anyone with a little knowledge about PDF. Like incomplete redactions, such hidden annotations have caused many embarrassments in the past, and will continue to catch folk out.

Preview’s bugs

Finally, Preview has had more than its fair share of bugs in handling PDF annotations. During my research for this article, Preview 11.0 (1069.7.1) in macOS 15.6 was generally well behaved, but did mangle comments added to a test document by PDF Expert and Adobe Acrobat. Preview has two behaviours that can appear disconcerting: that of its Highlights and Notes tool, and its use of versioning.

All Preview’s tools are single-shot apart from Highlights and Notes, the drawing pencil icon to the left of its popup menu. Click this once to apply highlighting to selected blocks of text, and to remove existing highlighted sections. Unfortunately when this tool is turned on, its own highlighting is so weak that it’s hard to see.

Overwritten files

Preview has a habit of saving PDF documents automatically when closing them, without any warning. If it has just mutilated an annotation, for example, you might assume the original file has just been overwritten and lost. However, Preview saves PDFs using the macOS document versioning system, so you can always recover the previous version.

This might at first seem an impossible task: use Preview to restore that old version and it will repeat its mutilation, defeating the purpose. Yet the original PDF editor won’t have access to previous versions, as it doesn’t use the versioning system. The solution is to use Revisionist, or Versatility, which can save the original as a separate document.

Key points

• Annotations are a central feature of PDF, come in many sub-types, and can be complicated as they can be expressed in different ways.
• The macOS Quartz PDF engine transforms them into PDF-1.3, which makes them simpler and more explicit, so they can be saved ‘flattened’.
• PDF format mixes document data with metadata and annotations.
• Few PDF editors offer to remove all annotations, and there’s a risk of some remaining hidden from view, but still remaining in the PDF source, potentially causing embarrassment when they’re discovered.
• Preview’s earlier bugs in annotations have improved, but it can still mutilate those made by other PDF editors.
• If Preview saves a mutilated PDF, you should be able to recover the previous version of that file using Revisionist or Versatility.

To make its graphical interface work, the Mac needed a high-performance graphics system, for which the late Bill Atkinson (1951-2025) and Andy Hertzfeld designed and implemented QuickDraw. When it came to driving printers, though, Steve Jobs licensed the new page description language PostScript from Adobe, where it had just been developed by John Warnock (1940-2023), Charles Geschke (1939-2021) and others. PostScript is a stack-based interpreted language that could take many seconds or even minutes to image a page for printing, so wasn’t practical for doing much else at that time.

In the early 1990s, as desktop publishing became dominant among Mac users and we were all sending one another faxes, several companies recognised the need for a universal document format that could display laid-out text and graphics. Among them was Adobe, where Warnock formulated the aims of what he then referred to as Interchange PostScript or IPS, and so led the development of Portable Document Format. It’s telling that the final sentence of his proposal reads: “In any event corporations should be interested in site-licensing arrangements.”

When the first version of PDF was released in 1993, with its Carousel reader app, it faced competition from other similar ideas, and Adobe found itself competing against products including Farallon’s Replica, and Tumbleweed’s Envoy that gained the support of WordPerfect, then a popular cross-platform word processor. PDF didn’t become dominant until Adobe distributed its reader app free, rather than charging $50 for it as it had initially.

For many years, the only way to create really good PDFs was using Adobe’s Acrobat Distiller app, costing $695 for a single-user licence. That ingested PostScript files, created on the Mac by printing to a file, and transformed them into PDFs that could in turn only be read using Adobe’s software. Although PostScript was by then a prerequisite for all publishing work on Macs, it wasn’t until 1996, when PDF reached version 1.2 in Acrobat 3.0, that it captured the prepress market, which it consolidated in 1998 with the PDF/X-1 standard.

This is Acrobat Distiller 4.0 running on Mac OS 9.1 in early 2001, showing a few of its bewildering array of options for turning PostScript files into PDF.

At the same time, John Warnock’s aspirations for success in enterprise markets were being realised, and PDF steadily became the standard for fixed-format electronic documents, with the support of the US Internal Revenue Service and Adobe’s free cross-platform Acrobat Reader.

When Steve Jobs established NeXT in 1985 he must have become the only person to have licensed PostScript from Adobe twice, as NeXTSTEP adopted Display PostScript as the centrepiece of its graphics, developed collaboratively between NeXT and Adobe. At the time many thought this to be a mistake, as PostScript isn’t as efficient a graphics language as QuickDraw, despite Adobe’s efforts to accelerate it.

When NeXT and Mac merged to form the beginnings of Mac OS X in 1997, Display PostScript was replaced with PDF as the central graphics standard for both display and printing, in what was dubbed Quartz 2D. This was first demonstrated at WWDC in 1999 and lives on today in macOS. At the time, Apple’s in-house PDF engine in Quartz was one of few, alongside Adobe’s.

Prior to Mac OS X, Adobe Acrobat, both in its free viewer form and a paid-for Pro version, had been the de facto standard for reading, printing and working with PDF documents on the Mac. The Preview app had originated in NeXTSTEP in 1989 as its image and PDF viewer, and was brought across to early versions of Mac OS X, where it has remained ever since.

This PDF shows Apple’s original iPod promotional literature from late 2001.

Adobe continued providing its free Acrobat Reader for Mac OS X, here seen in 10.0 Cheetah.

The full paid-for version of Adobe Acrobat provided an extensive suite of editing tools, here in Mac OS X 10.1 Puma in early 2002.

By Mac OS X 10.3 Panther in 2003, Apple was claiming that Preview was “the fastest PDF viewer on the planet”, capable of navigating and searching text within PDF documents “at lightning speed”. This worked with the Mac’s new built-in support for faxing, which rendered received faxes in PDF to make them easier and clearer to access.

This is an early Keynote Quick Reference guide from 2003, viewed in Preview.

At that time, Preview was also able to convert Encapsulated PostScript (EPS) files and raw PostScript to PDF, so they could be saved in the more accessible format, and printed easily.

This page from the 9/11 Commission Report of 22 July 2004 is being viewed in Preview.

Acrobat Distiller remained an important component in Adobe’s paid-for product, even though Mac OS X was capable of generating its own PDFs. It’s seen here in Mac OS X 10.4 Tiger in 2005.

This is Acrobat Pro in 10.4 Tiger in early 2006, showing its long list of supported export formats.

Since those heady days, Preview has been relatively neglected. Revision of both the Quartz PDF engine and its API brought a spate of bugs that only abated with macOS Sierra. Preview has adopted an uncommon model for PDF annotations that often doesn’t work well with other PDF products, but it has remained very popular for completing electronic forms. Then, in macOS Ventura, Apple removed all support for converting EPS and PostScript to PDF, most probably as a result of security concerns, and their progressive disuse.

Although rumours of the death of Preview continue to prove unfounded, it’s unlikely to feature again as one of the strengths of macOS.

References

John Warnock (1991) The Camelot Project, on the Internet Archive’s Wayback machine.
Laurens Leurs’ The history of PDF.