unlink error - don't know how to fix it

[eluser]überfuzz[/eluser]
I get an errorwhen I try to unlink a pdf-file.

A PHP Error was encountered

Severity: Warning

Message: unlink() [function.unlink]: http does not allow unlinking

Filename: information/schema.php

Line Number: 109

How do I set the permission to 777 or something arbitrary when I upload it? I get 644 all the time. Hence I can't unlink.