PDF have became the default file format for our everyday technology use. Download a file on the Internet, most likely it would be in a PDF file. Files email to clients for their signatures are most likely pdf files. Scan documents using a scanner and it would be save as an image file (.jpg) or .pdf. It would be better to save the file as a pdf than an image file because there are more options to compress the file and the ability to search the document especially if there are multiple pages.
Adobe, the famous software maker of Photoshop and Dreamweaver, was the company that made pdf popular by providing a free Acrobat PDF Reader. There are other free pdf readers out there, but currently the most popular is from Adobe.
Being able to open pdf files for viewing is useful, it is better to be able to create pdf files from common programs like Microsoft Word, Excel, etc. and that is where it is required to purchase pdf writers such as the Adobe Acrobat for about $200 or lower cost pdf writers for about $30+. My understanding is that Open Office, which is free does write pdf files and could be something to look at. I talked about Open Office on one of my blog and can be access here.
If you own an iPad or ebook reader device, pdf are supported but many of these devices uses their own proprietary formats. Regardless, pdf is a great format and a great investment in our business.