My university’s library has issues of many older journals currently only available on microfiche, which are 4×6″ sheets of film used to store analogue copies of pages. These sheets are highly compact and durable (if you ignore the fact that film is quite flammable) and while they sort of look messy, they tend to preserve contrast well and thus are quite readable.
When I request a paper available on microfiche, the librarian uses a machine which scans the microfiche and generates a PDF. Unfortunately, the resulting PDF is a negative of the text, which I find slightly difficult to read. Thus I have been using UNIX tools to invert the negative into a positive image. One solution that works is based on the widely-available ImageMagick. For instance, the following command does this at 300 DPI without any loss of quality:
magick -density 300 input.pdf -negate -quality 100 output.pdf
Just sharing in case this is useful for anyone else.