Printing to PDF with AVR Web apps has always been a royal hassle. Doing so required expensive, and challenging to configure, third-party print drivers; low-end print drivers don't work either because they are single-threaded or require registry access to assign a PDF file name at runtime. In the early days of Windows, registry access for such ad hoc tasks was easy but in these security-conscious times it's not an acceptable best practice (if, given your security policies, it can even be done at all).
Microsoft has finally come to the rescue by adding native PDF print drives to both Windows 10 and Windows Server 2016 (perhaps signaling that MS realized its XPS document writer isn't a PDF killer!). That's the good news, the bad news is that this PDF print driver is not present, nor is it available, for previous Windows or Windows Server versions.