I agree that the pdf show a lot of antialiasing artefacts while the imported image in Prime looks better.
I now used your Prime file and exported it to a pdf three times.
Using 'printer' "Adobe PDF Print" using standard settings created the smallest file, but also the worst one in quality. Of course the settings in Adobe print could be adjusted.
Using 'printer' "Microsoft Print to PDF" created a much better quality file at the cost of four times the size
Using Primes own "Save to PDF" created a file similar to that of Microsoft, maybe Prime is just using the same printer driver.
So the problem may not be Prime but rather the way you use to create the PDF?
How did you create it, which pdf printer software did you use? Maybe you can set options there to increase resolution, etc. Or you may give Prime' PDF export facility a try.
I attach the three PDFs mentioned above.
EDIT: Just looked at the properties of the file created by Primes export facility and, yes, it used "Microsoft Print To PDF".
EDIT: I played around with the setting in the Adobe printer driver (a rather old version) and choosed something like "quality print". I expected a larger file size, but the file got smaller in size and better in quality 😉
Its additionally attached.