As you probably know, Adobe Acrobat 7 is ancient and not supported on any modern operating system. I am not aware of this being a common problem - I've actually never heard of this when just blank pages are being added - so it's very likely that it's impossible for us to figure out what is going on without having access to the document. If you can share it, here is some information about how to share it: Share Documents via Adobe's Document Cloud - KHKonsulting LLC
This could be a font issue, ore a problem with a corrupt PDF file, or probably a number of other things.
How are you adding the three blank pages?