RPGs are an area where PDF publishing thrives. It lowers the barriers to publishing and can (in theory) result in lower consumer prices. Still, many folks prefer paper. Lets see how they stack up to each other.
Cost - PDF versions are generally the cheaper alternative, which is always a selling point.
Portability - Depends on what you read your PDFs on. An iPad, tablet or ebook reader can store hundreds of PDFs and are extremely portable. So can your desktop, but that isn't very portable.
Marking Up - While there are programs and applications that allow one to mark up PDFs, it's easier and quicker to mark up / make notes on a paper copy.
Bathroom Reading - Paper. It just doesn't seem right having my iPad in there. Besides, reading in the tub is definitely a paper situation. Less to risk with accidental immersion.
Ease of Flipping Thru - Unless the PDF is well bookmarked, it's way easier to flip thru a paper copy then a PDF copy.
Less Tangibles - You can throw a book across the room in disgust with generally less damage then you can an iPad or laptop. PDFs are much easier to make backup copies of. Some people just prefer the feeling of a book. PDFs take up much less storage space. PDFs can be corrected by the publisher, paper books may get an errata sheet.
I'm sure there's more to add to the list...
Introducing Night Shift - I had something planned for today, but it will have to wait till later. Busy day at work. So here is something I have been working on. So folks, to coin...
6 minutes ago