The whole point of a document in Portable Document Format (PDF) is that you’re not supposed to be able to edit it. PDFs were invented by Adobe to solve a common problem in the early days of computerised typesetting: documents would print differently depending on what computer system you were using, what fonts you’d installed, and what type of printer you were sending the job to. To get around this problem, Adobe invented a format that contained an exact representation of how the document would look when printed, with graphics and any unusual fonts embedded inside it. Creating a PDF was supposed to be the final stage before your document went off to print; the ability to keep editing it was sacrificed for verisimilitude. It’s best to think of a PDF as a picture of your document.

These days, more and more people are creating PDFs, and they’ve become the default means for distributing formatted text. It’s always been possible to print a PDF from Word on the Macintosh, and Word 2007 for Windows introduced an add-on that allowed the same thing. So PDFs are everywhere, and it’s natural enough to want to do things to them. Thus, there’s a demand for ways to get around Adobe’s locked-down file specification.

Buy Acrobat Pro

Adobe’s free tool for reading PDFs is Acrobat Reader. If you work with PDFs a lot, it’s worthwhile getting the professional version of this. Acrobat Pro allows two main things: minor edits, and annotation. It’s not cheap, though: about NZ$250 at education prices.

Annotation is very handy when editing or commenting on someone else’s manuscript. You can add sticky notes or pop-up comments to the text, highlight or underline it, scribble on it, add call-outs and so forth—more or less the same things an editor would do to a printed manuscript.

The editing tools are intended only for minor edits: the rule of thumb is that you shouldn’t try to make any changes that would cause text to reflow from one line to another. So the Touchup Text tool lets you add or delete a few words, and you can rearrange elements with the Touchup Object tool. Although Adobe strongly suggests you go back to the original document for substantial text changes, this minor editing is enough to correct typos.

Free Alternatives

There are some things it’s not necessary to buy Acrobat Pro for—indeed, it’s always worth Googling to see if a free online solution to your problem exists. For example, a common task is adding a covering letter to a separately-created grant application, and there are a number of simple Web-based PDF editors to do this, like MergePDF.net. Likewise, if you want some of Acrobat Pro’s annotation tools for free, try Skim (Mac only).

Wikipedia has a reasonable list of PDF editing tools, both installable and Web-based. (Note that one Windows application, PDFCreator, installs annoying malware and should be avoided.)

Going Backwards

In an emergency, it’s possible to convert a PDF back into an editable document. There are numerous online services that will convert a PDF to Word, though they don’t all work very well. I would recommend finding a few via Google and trying them all. If the PDF was created from an electronic document (not a scanned one), you can also use the Select tool in Reader or Acrobat Pro to select and copy text into a Word document.

So what do you do if you’re starting with a scanned document, like most PDFs of older journal articles? A scanned PDF is just a picture of the page, not text. Acrobat Pro might save the day, though: it has Optical Character Recognition software built in, which can work through the scan and interpret letters and words, ending up with a PDF you can select and copy text from. OCR software used to be amazingly expensive, so it’s nice to see it included in Acrobat Pro. You may want to run this on batches of your PDFs; one side-effect is that it will reduce the file size.

Check out this ProfHacker post about different ways of annotating and filing PDFs.