Are you having problems in opening/reading pdf files stored on the Web?
Users of Mozilla Firefox reported such problems and it happened to me too, with my own pdf files (the free quilt design I send you every month- more than half of the file had blank pages).
It happens because Firefox has a built-in PDF viewer that allows you to view PDF files found
on the web without a plugin. The built-in PDF viewer is enabled by
default but it is still under development and works fine for simple PDF files, but a
lot of advanced features aren't supported yet and there are still a lot
of bugs open.
If this happens to you too, you could try to open the files in other browsers (Chrome, Internet Explorer...), just to check that the file is OK.
Then read the article below and switch to another plugin ; go to Firefox options > applications & scroll down and
define the default action for the portable document format (pdf).
Here is the article:
Disable built pdf viewer and use another viewer
I hope this helps, I spent a few hours trying to understand what is wrong with my files :)