[Frank Cox]

You can use https://imagemagick.org to fix your pdf file.

You might have to export each page to a separate image first for the best results.

Your workflow would look something like this:

pdfimage myfile.pdf i

mogrify -sharpen 0x1 i-*

You might want to experiment with the sigma value for the best result. It can be any floating point value from  .1  for practically no sharpening to 3 or more for severe sharpening.  0.5 to 1.0 is usually rather good.

You can make the images back into a pdf file using various methods; I usually do it like this:

for i in *; do convert "$i" "$i".pdf; done
pdfunite i*.pdf endresult.pdf

convert can create the pdf file from all of the images at one time but sometimes it seems to overrun the available memory when it's doing that so it's usually better to just make each page into an individual pdf and combine them at the end as shown above.