I had some problems with some of the answers not fitting mine's. Specifically:
What I want in a pdf
There are things I want from EVERY product, including a pdf. Things like being just the thing I want to run/use, like being of engaging and interesting, like having great player handouts and making for a memorable experience, like being easy to use...
Then there are the things that are specific to pdfs. Which are being cheap, lacking black- or patterened-backgrounds (for ease of printing), and for large pdfs bookmarks and hyper-links.
Since it was a text-box, I said it all there.
Does Art Matter?
High quality art improves the chance of purchase, but no art is far better than poor quality art. Player handouts, monster pictures, and so on are more of a lure than GM-secretive art (although a slick graphic design and pictures do improve chance of purchase).
Maximum Size for PDF
Over 100 or so. I wouldn't print a 100 page book, but I AM printing out a 60+ page adventure this evening.
It isn't so much a matter of size as of use. Some products (adventures) I prefer as pdf, others (rulebooks and settings) I prefer as hardcovers or dead-tree. A very small product is always preferrable as a pdf, as it costs next to nothing to print it and purchasing it in dead-tree version is not practical; otherwise, use not size determines the preferred medium.