It's a blackberry restriction, not a carrier one. In BES you can even configure it. Get Repligo Reader which is the best pdf reader money can buy for the blackberry platform and drop your pdfs to SD.
I use it daily and read easily PDFs from a few MB in size to encyclopaedias and research reports that reach up to 250MB in a single file. Never had a problem.
From the option menu select Reading View and the page is automatically formatted to fit the screen. They offer a free trial so you can test yourself if it works for you.
Hit
RepliGo Reader with your bb browser and give it a try.