I like CutePDF. It works really well for me.
Open the file (it can be anything that you're able to print to like a normal printer). It can be a word document, image, etc. Then click on File --> Print. Instead of selecting your default printer, choose from the drop-down menu, "CutePDF" instead. Click Print, and it'll ask you where you want the PDF file to be saved.
http://www.cutepdf.com/Products/CutePDF/writer.asp