This hint shows how to load a one-page PDF into a word processor and place an image or text over the PDF.
I regularly get PDF email attachments that I need to sign and return to the sender. In the past, I've had to print the document, sign it, and scan it to email it back. I was looking for a way to do this without having to print the document (or use Adobe Acrobat), and this is what I figured out.
First, scan your signature and save it as a decent-quality image (.gif works, as does .jpg and .png). Keep this image for any future need. Second, open a word processor (MS Word and Pages both work) and create a new document. Drag the PDF into the document, and it will come up as a smaller document, with adjustment handles. Under 'wrapping' in the formatting palette, make sure the document is in the background, or floating (not Inline). Then, adjust the size of the image so that it exactly fits on the page (it's easier to do this if you reduce the page window to less than 100%).
Once you have the PDF in the background and sized appropriately, you can place images or text boxes wherever you like. Then, simply Print to PDF, and you'll have the merged PDF. The PDF text will remain selectable and the PDF size should not change appreciably (depending on the extras you add).
If you want to do this to a multi-page PDF, it is more time consuming, but doable: first 'Print to PDF' the page you want to change as a single page PDF, and then follow the instructions above. Finally, merge the new PDF page with the old document by either printing the other pages as two separate documents and then using Preview (as in this hint), or by using a PDF Merge application such as PDFMergeX (now PDFGarden) or other freeware.
[crarko adds: I tested this, and it works as described.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110108155032926