HP 4050 Printer won’t print PDF files.

I have an HP 4050 LaserJet printer which is attached to a TCP/IP network via a HP JetDirect card. The print server is a Windows NT 4.0 Server. Client PCs print to this printer fine until they try and print a PDF file. Then the document ties up the printer saying “User intervention required.” The OS on these computers vary from Win98, Win ME, and Win 2000. They all have the same problem which leads me to believe it is either a printer problem or a problem with the print server. The only commonality that I have been able to find is that it is only a problem when the users TRY to print PDF files.