Many people use different programs to make documents. For example, if someone uses Word Perfect to make a document and send it to me, I won't be able to open and view it unless I also have the Word Perfect program.

I make many documents for my clients using QuarkXpress, but my clients don't have that program. So I turn the documents into a pdf file (adobe acrobat) so that they can view it.

Adobe Acrobat Reader, a free program which you probably already have, allows you to open and view any pdf file. But to make a pdf file from your document, you normally have to purchase Adobe Acrobat. The link above apparently allows you to make your own pdfs for free.

Did I just confuse you more?