Handle PDF files import/export

[Foul-Tarnished]

Motivation
This would make upscaling pdf ebooks easier (especially old ones that were scanned and are not available as "proper pdf")

Description
Make the "Load images" and "Save image" nodes work with PDF files.
It should read every pdf pages, and save all images in a single pdf

An even better approach would be to upscale the images in the PDF, but not touching the text, and then remaking a pdf with the upscaled image and original text