try it?
I use the 770 primarily as a Reader. I have hundreds of pdfs and several hours every day i read on the 770.
The built-in reader has a good rendering speed if you consider the slow CPU, i give it that, but it craps out very fast if your pdf contains pictures or is a little bigger, then it runs out of memory or freezes/reboots the 770 (which is not acceptable).
Evince has some nice features which the builtin doesnt (contiunous mode, better fullscreen-mode) but it is slower and way more unstable.

I tried it, it sucks, and thats why i had to convert 90% of my pdfs to jpg-galleries, they are 10 times bigger in filesize but with a 2GB Card its not a problem and i can view them faster and stable in opera then i could otherwise.