You could try using Firefox or Chrome on Mac OS 10.5.x instead of Safari. It may work. Mac OS 10.5 is not a supported environment: https://www.acrobat.com/welcome/en/misc/system-requirements.html
Alternatively, you could access the ExportPDF service through Adobe Reader X (not XI) tools pane.
Please let us know if you have any questions, we'll try to help get this working for you!