The situation is that one receives a fax as a PDF document, but needs to sign it or make other updates, and then return to the sender as another fax. Without a fax machine, how does one print, sign, and fax?
A program called NitroPDF Reader, a free alternative to Adobe Reader, includes functionality to "sign" PDFs, as well as to complete and save PDF forms (the ones with a box for each datum to be entered) and type data into PDFs (when the PDF doesn't have boxes, but the user needs to enter information).
Nitro explains the process to create a signature here, using a scanner or a phone with a camera. Once created, a signature can be added to any PDF document (unless the person who created the PDF has secured it, which shouldn't happen with a fax).
But wait! There's more! With NitroPDF reader, one can create PDFs. The program installs a "virtual printer," allowing the user to print from any application to create a PDF. Need to send an Excel spreadsheet as a fax? "Print" it to NitroPDF, and use Sonic.net's "Send A Fax" tool. In many cases, it is possible to drag a file onto the NitroPDF icon and have the file opened as a PDF, ready to save. (I often "print" web pages to PDF to save to read later.)
NitroPDF Reader is free to download and install. Works with Windows XP, Vista, and 7.
(I've tested sending a document "signed" using NitroPDF reader, through Sonic.net's Send A Fax service, and it arrived at the destination looking exactly as intended.)