pdf is simply a file format where you need a certain type of program to open it. For example, most documents need Microsoft Word, Word Starter or other similar programs to open the file. And most computers come with the program~it's readily available. Well it's the same with a pdf file. If you have Adobe Reader on your computer, you can open pdf files. If you don't have it, it's free to download off the internet. :) My warning is that when I started listing, I had computers in mind, not cell phones so if you're using a cell phone to get access to Listia, you need the Adobe reader mobile app which I don't think is available for ALL cell phones. You'd have to look into that. :)