Confluence cannot resolve the file path of the 'temp' directory in the home folder properly if it is modified in the confluence.cfg.xml.
This leads to a 403 - Forbidden - page in Confluence when attempting a PDF Export.

E.g. Default value:
<property name="webwork.multipart.saveDir">${confluenceHome}/temp</property>

Changed value:
<property name="webwork.multipart.saveDir">/home/foo/confluence-temp/test</property>

The result is this URL and 403 page shown:

http://localhost:8080/download//home/foo/confluence-temp/test/export_06192007_133711/ds-32787-20070619-13_37_11.pdf

Checking the proper path with HttpLiveHeaders, the correct path should be something like this:

http://localhost:8080/download/temp/export_06192007_133711/ds-32787-20070619-13_37_11.pdf

Confluence should be able to update the path with this change. Consequently, it can be fixed by reverting the above webwork.multipart.saveDir back to ${confluenceHome}/temp.

This issue can be reproduced by using the symbolic link as well.

Workaround solution:
On Linux, you can use the file system bind via "fstab" to bind the temp folder with another temp folder from another file system.