Dr Mike J Smith
Loads of stuff is sent, received and archived as PDF. Its a great format originally derived from postscript (by Adobe) designed for bridging the gap between designing a print job on a PC and delivering it to a printer for output. Since then it has evolved considerably to include audio, video, 3D, layers, meta-data, digital signing and form filling to name a few. And since Adobe opened the format up there have been many vendors providing PDF readers, writers and editors. Obviously Adobe has its flagship product Acrobat, but Foxit remains my favourite for small footprint, rapid rendering.
In terms of writing files there are a few open variants that are worth considering:
FreePDF-XP: if you can get past the impenetrable German download page, this is a very capable and useful product.
DoPDF: nicely configurable PDF printer driver. Plenty of options, way over and above the simplistic PDF export in MS Office.
If you fancy a nice combination then FinePrint allows you to accumulate and edit print jobs, whilst PDF Factory produces the output. Paid for, but excellent apps.