Comment 7 for bug 1796902

You also need to add it to gui2/convert/pdf_output.py

Thinking about it, it would probably be better to implement this as an arbitrary expression rather than as a shift. That way one could do more sophisticated processing of the page number. For example,

--pdf-page-number-map "if (n < 3) 0 else n - 3;"

it could be implemented as a javascript expression, evaluated using evaljs for safety.