You might want to either post a sample document or a pointer to one. You will need to look at the HTML and see how the tables are specified.
If they are laid out with widths in pixels that may be your problem. You might have to remove the pixel specification and change it to percentages. If memory serves the PRS 505's screen is 800 pixels tall by 600 wide. This might create new problems. If the reports you want to look at are consistent though, it might be worth it as you could do a search and replace once you figure it out. If you have never meddled with HTML before, you might look at:
http://www.w3schools.com/html/default.asp
You might change the screen orientation to horizontal. That would be wide enough, but your tables will be not be displayed as a whole top to bottom.
As much as I love my 505, it might be much easier to display such reports in a tablet computer. It would also display PDFs without conversion in the size they were designed to display in.