Editing a PDF--font and custom encoding issues

I am trying to edit PDF text using Professional v8. When I highlight the text using the text edit tool I get the following error message.

"All or part of the selection has no available system font. You cannot add or delete using the currently selected font."

When I try to change the font I get the message.

"The change to a different font was not done because the chosen font and the font encoding in the document differ and could not be resolved."

What exactly is the font encoding? The encoding for the font in the PDF I want to edit says custom encoding. What is custom encoding?

Is there someway to get around this problem.

The document was created on a Mac and I have been using a PC. I tried to edit the document on a Mac and it would not let me edit it, but I did not get any error messages.