What is the problem you are wanting to solve?:
In T256848#6272386 it was noted that page scan images from a PDF, had improved quality when rendered at a higher DPI.
(I am therefore thinking the original scans are at a higher DPI such as in 300 or 600 dpi (typical in imaging and print applications) vs 150 or 96dpi (typical for screen displays)
There is not currently a way to specify the DPI to be used from the File: page, and thus external tools ( like Ghostscript) used for rendering the initial output, later used by Thumbor to generate scaled images, do not have any means of getting this information, directly from uploaders at present.
What's the feature you want:-
*The ability to specify a DPI value on a File: (or Index:) page, and have that value taken into account during the generation of thumbnailed or scaled images based on page content in a PDF file. ( User specified DPI cwould also potentially be useful for DJVu content, but I have no knowledge whether the decoder used on Mediawiki installations allows it to be specified.)
The DPI option could ideally be stored as a 'property' for the File:'s entry in the image links table. and thus should ideally be implemented as a 'modifier' template for use on sites like Wikimedia Commons and Wikisource.