I have come across this a number of times, most recently on a post on Community.QlikView.com, so I thought it was worth sharing.
If you have any type of printing enabled in QlikView, the QlikView Desktop and Plugin will render the printing and submit to the relevant printer. This also gives facility for Print Preview. This facility is not available in Ajax - a limitation of the thin client.
To have printing in Ajax, any print jobs need to be rendered on the server into PDF and then presented to the user as a download.
The problem occurs if the IT department has "hardened" your QlikView Server. One of the common steps in hardening is to disable the Print Spooler Service. This is often built into a Group Policy rule also.
The problem for QlikView is that during installation of QlikView Server, the installer attempts to install the PDF XChange drivers. This step will fail if the Print Spooler service is not running.
Once you have sorted out the Print Spooler Service, you don't need to re-install QlikView Server. There is a PDF driver available as a separate download in the QlikView downloads area.