I think this is a bug (but it also might be a feature :)
If you try to print a web page located at
doPDF defaults the name to "pagename.php?var1=value1&var2=value2" instead of "data.pdf"
This is a browser thing, not something we can control via doPDF.
For example, Internet Explorer will send as filename for doPDF to use the url of the printed page. So in your example the name of the pdf would be something like www_example_com_pagename.php_var1=value1.pdf (it replaces some characters that might not be allowed by Windows when creating the file).
On the other hand, FireFox sends as filename for doPDF to use the title of the page.
So there's not much we can do.