1. What's exactly the license of FPDF? Are there any usage restrictions?FPDF is released under a permissive license: there is no usage restriction. You may embed it freely in your application (commercial or not), with or without modifications.
2. When I try to create a PDF, a lot of weird characters show on the screen. Why?These "weird" characters are in fact the actual content of your PDF. This behavior is a bug of IE6. When it first receives an HTML page, then a PDF from the same URL, it displays it directly without launching Acrobat. This happens frequently during the development stage: on the least script error, an HTML page is sent, and after correction, the PDF arrives.