I use PDF995 it basically works as a printer so all you do is plot to that printer and it comes out as a PDF, this is the same for any file type. Anything that you print to the PDF printer comes out as a PDF file (even if you highlight some text on a web page for example).
It is free software but you get a "sponsors" pop up unless you pay $9.99 (one off fee) if you pay the higher amount of $19.99 you get a couple of other bits of software (PDFedit) that allow you to manipulate PDF files such as combining different files into one so that you could take a full set of drawings and then print them to one file rather than lots of individual ones, very good for say a set of isometric piping drawings for a section of plant.
It has also proved useful for my expenses claims, I can scan all the individual receipts and then combine all the scans into one file and email the whole lot to the USA for payment, previously it was done by hand with all receipts stapled and then posted. Now they will accept one PDF file so everything is now electronic.