When I run a .pdf file from a DVD with Firefox it reads it like the picture shows:
Why is this happening?
This is the binary content of your .pdf file which is simply shown as text. You can change the behaviour by changing the action for pdf files in settings -> applications -> Portable Document Format (pdf). Just set the action to "Preview in Firefox" for the integrated pdf reader or "download file" or whatever you like to do with the pdf file.