No right to open file once copied on the desktop
There are known issues (Apple-created bugs) with several revisions of OS X 10.5 Leopard Client and AFP file access, including permission problems and in certain cases even the potential to lose or overwrite files completely.

There is no work-around for these bugs, they are in the operating system itself and can be reproduced on 2 freshly-formatted &amp; installed Macs running Leopard 10.5, and most or all of the later revisions, i.e. 10.5.1, 10.5.2, etc.

Your best solution is to be sure to update all 10.5 Macs to 10.6.8 Snow Leopard. Failing that, updating everything to 10.5.8 (the last release of Leopard) may solve your problems.

Here's a list of references that go into more detail:

http://support.grouplogic.com/?p=1508

https://discussions.apple.com/thread/1423847

http://www.insanelymac.com/forum/topic/67625-preping-for-105-permissions-issues-with-afp-shares/

http://lists.apple.com/archives/macos-x-server/2010/Dec/msg00267.html

http://www.mailinglistarchive.com/html/xsan-users@lists.apple.com/2008-07/msg00409.html