Print to PDF Settings

Hello I have a collection of thousands of PDF books. These PDF files are mixed in times from the start of PDF days to now. So with that said. Ubooquity scans properly and imports only half of my PDF files.(I'm having a hard time trying to figure out why some PDF  files work and others don't) They all show up for download. But the built in web viewer reader stays stuck at "loading" or Pages show up like they are corrupt but any offline PDF reader reads these files fine.

So I have been looking around for a good PDF printer so I can somehow convert these PDF files back to a PDF format that Ubooquity supports. Problem is every PDF printer I try have a tons of settings and I am not sure what the "optimal" quality settings would be to get as close to one to one copy? Anyone have any ideas. I am able to convert but not sure what I'm doing. In some cases taking 30 minitues to convert one document. Again I think there is a problem here when the source is a 12 meg PDF file that Unooquity will not display. I take this file and Print to PDF and the file sizes get to  over 700mb a file. Crazy!! I Then try compression settings in PDF printer. (right now using Bullet) Then the PDF export is not so bad but still like 70mb instead of the native size of 12mb.  Ubooquity now reads the file fine But the view quality is so horribly bad. The text is all pixel like a highly compressed image file.

I was just wondering if anyone could recommend the best "Settings" for a PDF file. I must be doing something wrong. Any info hints recommended software would be appreciated.