Open file attachment - PDF appearing as text - D6 with private method or upload private Module

Hi,
I'm using Drupal 6.4.

Using the "private download method" or the "private upload module" the uploaded files get the path /system/files/..... and are hidden for not authenticated users. That works fine for nodes but not for uploaded PDF..

Only in Firefox (2.x or 3.x) uploaded PDF is opened as text when downloading the file (no window apear).
If the download method is private, everything is OK!

Can´t find a solution. Please give me a hint!!!!!