Ollie,

            I've used this program primarily with JAWS, and it is not accessible in its entirety, but it is for performing OCR and saving the files.  The command CTRL+SHIFT+C is noted in the NVDA keystrokes documentation as being special use for the program Poedit, and if this key sequence is not turned on/off based on whether one is in Poedit that could be your problem.  In which case you'd have to use NVDA+F2 prior to hitting CTRL+SHIFT+C when you're in PDF-XChange Viewer to pass the keystroke through.  I cannot speak specifically to the interaction between NVDA and PDF-XChange reader in its entirety.  I'm using NVDA 2016.1 and when I'm in the program CTRL+Shift+C brings up the OCR dialog and it is announced as such by NVDA.  When I use ALT+F to open the File Menu and use down arrow to traverse it nothing is announced, but if I move my mouse pointer over the various menu options they are.  At first I thought this might be a result of the NVDA default in mouse preferences of having mouse tracking on, but even when I turn mouse tracking off I get nothing announced in the file menu via keyboard navigation. ALT+F,S will save a copy with the OCR embedded if you've done OCR and it's complete, even if NVDA does not announce it.

            The only thing I recommend screen reader users employ PDF-XChange Viewer for is the OCR function.  The only things I've taught are opening a document, triggering OCR, and saving a document and all of those things are announced in JAWS, but NVDA is not on the menu itself but when the respective dialogs for open and save are brought up via ALT+F,O and ALT+F,S those are announced.  For reading stick to Adobe Reader (or your own favorite if you have one).

Brian