Can you upload the files here for us to test?
Also, is it happening more than this one file?
What steps did you take to get you at this point?
What font is the spreadsheet using?
Have you attempted printing to PDF instead of save as PDF?
I also use inserted excel spreadsheets in drawings and found that printing to PDF instead of save as PDF does not pixelate the text in the spreadsheet.
Have not tried printing to pdf. I do not get that option, and IT does not like us to download anything.I used to use Primo pdf at a previous job.
I just confirmed it on my end (2018 SP3). The font in the spreadsheet is Arial. So, it's not a font issue. This is a SW issue and I consider this a bug. Can you send the files to your VAR and have them create an SPR? If you do that and post it back here, I'll vote for it.
I use Save As PDF very often and if I need an excel sheet sometime in the future, I'll want it to work correctly.