I think an issue is that the text will not wrap after you enlarge it beyond a certain size and you will have to scroll back and forth. That is a general limitation of many pdf documents. It is possible that the Kindle Fire was reading a 'book' type document which allows lots of resizing.
I really like pdf reader as suggested by the other reply.
Exactly. As said above, if you have a EPUB or MOBI file you can read it on the Playbook using the appropriate e-reader app (try Book Reader). You can scale up/down the fonts just like you would on the Kindle Fire.
If it is PDF file you cannot resize the text other than pinching to zoom. The Kindle Fire would be exactly the same with PDF files.
I find for reading magazines in PDF form, I have to use the Playbook in landscape mode and zoom to fit the width of the page.