If you find some PDFs are too slow to operate on your iLiad, i.e., more than 5 seconds to turn a page, you are very likely having a poorly generated PDF with Type3 font.
Type3 fonts are bitmap fonts, and the iPDF software on your iLiad doesn't handle them very well (a problem with the poppler library it uses). One evidence is that they open fine and very quickly in Acrobat Reader on your PC.
To test if a PDF is using such bitmap fonts, open them in Acrobat Reader and check the embedded font list, or zoom in to 1000% and see if the characters have rough edges. Type1 fonts or TTF fonts should have very smooth edges.
If you are indeed having such PDFs, there is a way to fix them by replacing Type3 fonts with their Type1 counterparts.
(The following is for Linux folks, and if you are using Windows or Mac, you can also try it if you have Perl installed but I don't guarantee success)
First, you need to obtain a PS file for the same PDF. Many tech papers offer both PS and PDF for download (e.g., from citeseer). Or you can convert PDF to PS file by pdf2ps under Linux.
Second, you need to use a tool (written in Perl) called
pkfix. It's able to fix the Type3 fonts in many PS file with Type1 fonts.
pkfix doesn't work on all PS files, and if it fails, there is also a tool called pkfix-helper (on the same page) that is able to figure out which are the matching Type1 fonts by comparing font matrix. You may use the pkfix-helper first, and then pkfix.
The last step is simply to convert PS back to PDF. You may do this using ps2pdf under Linux.