Tuna,
A LaTeX install can easily be in the 10s of MBs (see my earlier post and links)
LaTeX ran well on machines w/ 25 and 33MHz processors (see Alan Hoening's _TeX Unbound_ for a paean to running it on a NeXT Cube)
A very nice front-end for LaTeX is LyX ---
http://www.lyx.org
If .pdfs aren't being rendered correctly on your PRS-505 that's a flaw in the renderer --- contact Sony so that they can fix it.
William