If you open a PDF with xournal it will convert every page to PPM (uncompressed image format) and therefore this needs A LOT of RAM. On my Talbet-PC i need up to 400MB for a 100 page PDF (i use a higher DPI setting for pdftoppm though).
This is certainly the cause for crashes/reboots of the N770/800.

Dont know about this issue with evince, but AFAIK evince depends on poppler and poppler includes the pdftoppm command.
Maybe the xournal package, assuming it does not have poppler as dependency, comes with its own pdftoppm version and doesn't work anymore then with the one from poppler.
Just guessing, i can't confirm or not confirm anything.