Lacking Natural Simplicity

I like to stick to Free/Libre (as in freedom) and Open Source Software as much as I can, but I haven't found a open source program that lets me add text on existing PDF documents yet, so I use a commercial but free (as in costs nothing for personal use) program PDF-XChange Viewer, which can be downloaded here.

Unlike most of the other commercial but free programs that allow you to add text to existing documents, PDF-XChange Viewer doesn't add any nagware watermarking to the document to indicate that it was produced with the free version, so you can use it fill out forms in PDF documents that don't have electric form fields, which is very useful on long forms.

It's not perfect — I've had trouble with it on a few PDFs that seemed to have pathological defects — but overall it works well.

I'd still rather have a Free/Libre or Open Source program, though.