- too low resolution for the texts
- noise, mostly JPG compression artefacts (= grey extras around black areas), can be partly generated by imgur
You can reduce some of the noise and increase the contrast with photo processing software, but these cannot guess what the text is - human or artificial intelligence is needed. The result from photo processing will be more difficult to read altough some contrast and solidity can be introduced. I tried it.
You must rewrite texts if you want to increase the readability. Surely some advanced pattern matching software could find what the texts most probably have originally been, but I do not know how one can get such tools.
Photo processing: Try noise reduction and then increase the contrast with the curves tool. If that does not help, restart with higher resolution and in all phases without making JPGs. Have TIFFs, PNGs, PSDs or another non lossy format.
ADD: The PDF which you linked later is good. Open it in Illustrator and recompose it there. It seems to be left editable. Often that's not the case.
You can copy and paste parts to a new Illustrator work. Or use Indesign. Forget Photoshop. Ask printhouse what you must bring to them - CMYK, print marks? Office printers do not need those specialities.
If you still need bitmap mode processing, then have at least 300DPI or more DPI and do not allow anything nor anybody trash the result with JPG compression. Save as JPG with high quality level can be acceptable, but you probably gain as much space saving with compressed TIFF. Packbit compression is useful in TIFFs if there's large one color areas.
I tried to read your PDF also with free and low cost vector mode software. Inkscape and Affinity Designer both gave far worse result than Illustrator.
Actually Inkscape could open the PDF with Poppler/Cairo import mode, but everything was grouped in an extremely complex way. Extension > Arrange > Deep Ungroup opened the knot, but now all pieces, even letters in texts were separate objects. All texts were right looking but uneditable curves.
It can still be usable, if there's no need to edit texts.
Inkscape opened the PDF also in internal import mode with PDF fonts (=texts stay readable and editable), but bizarre groupings made editing impossible. Deep Ungroup failed.
Affinity designer replaced included fonts without asking. Here and there were unreadable texts as a result. The images were good, but unreliable text import made the result unusable. A guru perhaps could put the fonts back, but I do not know how.