This is usually related to font problems: The font does not have a glyph for a certain character, so it just displays an empty rectangle. The first thing I would try is to load a file that is not form the same source as the field you've tried to far. One option would be this sample file: http://khkonsulting.com/files/LoremIpsum.pdf Download this file to e.g. your desktop and then open it from there. Do you see the rectangles as well? If so, I would try to repair the Acrobat installation. On the Windows version, you can find the Repair option in Acrobat's Help menu. If that does not fix your problem, you may want to look at "Use local fonts" preference. To access this, open up Acrobat's preferences, then go to the "Page Display" category, and change the setting for "Use local fonts". Does that change anything in how your documents are getting displayed?
I did try those items and it did not work.
Do any other applications show font problems?
Which version of Acrobat are you running, and on what operating system?