🚀 go-pugleaf

To expand on this, since I've been fighting the same issue:

The plugin doesn't receive any of the HTTP header information sent to the browser, all it works with is the literal URL webpage file name. With the 'Save a Copy' feature, it's smart enough to convert that filename into a .pdf extension at least. The 'Email a Copy' feature isn't quite so bright.

The only workaround is to setup your web server itself so that the script creating the dynamic PDF is using a .pdf extension itself, so in short you need to change (as an example):

showPDF.php
to
showPDF.pdf

and then configure your webserver so that the .pdf extension is interpreted through the correct scripting engine.