BlackBerry Curve 8520 already include free applications for viewing documents (.doc), spreadsheets (.xls) and presentation slides (.ppt). However, it doesn't natively include applications to view PDF files.
A paid application called "RepliGo Reader" can be used for this purpose. A 10-day trial is available from:
The BlackBerry App World:
Menu> BlackBerry App World.
Search App World for "RepliGo Reader".
Select "RepliGo Reader" application.
Press on "Download".
The application will be stored in the "Downloads" folder.