1. PDF's have become a nasty vector for exploits. Thank you Adobe. Need a viewer that can address this problem. My suggestion: a "viewer" that decompiles PDF's to Postscript, deletes any potentially harmful Postscript and then recompiles to a simple, safe early version PDF, all in one shot. I'm working something like this for myself using Ghostscript, but someone with more skill really needs to tackle this problem.

2. Ever tried to read/skim 100's of PDF's in rapid succession? (A common task if you are doing academic research.) It's near impossible. By comparison to reading text documents or viewing images, reading lots of PDF's is SLOW. Need a viewer that can view PDF's as fast as we can view text documents in pagers like less or images in viewers like feh. My suggestion: a viewer that extracts each page as an image and then views the PDF's as a series of images. (Essentially making reading them the job of an image viewer.) The trick is getting the sizing and resolution right, and dealing with hundreds if not thousands of extracted images.

PDF is great for printing. It also looks great on a screen (e.g. for casual, occasional reading, or presentations to an audience). But for reading lots of academic papers or any sort of document in bulk, PDF is absolutely terrible.

Postscript development is really an underappreciated area, I think. How does Apple achieve such lovely text on their displays? I could be wrong but I have always had a hunch their expertise in Postscript plays a role. They were once Postscript pioneers... remember the LaserWriter?