This permissions correction will be in the next release version of UBB.threads. If anyone needs this fixed right now and cannot wait until the next release, the fix is simple:
in /scripts/showflat.inc.php
FINDREPLACE WITHif ($ext == "pdf" && $userob->check_access("forum", "CAN_DOWNLOAD", $topic_info['FORUM_ID'])) {
thats all.
Outdoorking - thank you for this