My guess is Word can easily be edited; whereas, PDFs are not as editable (haha)...you have to have the right Adobe software to actually edit PDFs. That would be my guess.
Anybody can make PDFs using one of the many PDF-making software options out there. I use CutePDF. This basically creates a printer on your computer. So you would create what you want in whatever program you want (Word, Excel, PowerPoint, etc.) and "print" to the CutePDF printer, and it creates a PDF out of it. Basically, it would be like printing it out and then scanning it, but it cuts that part out and creates a clean "scan" of your document. And if you ever need to edit it, you just go back to the original file (in Word, Excel, PowerPoint, etc.), make the changes, and then "print" again. Best part is that the software is free and legal.