Okay cool. So it's something that mimicks a printer. In which case I think it might be an issue with that PDF driver that's causing the problem.
So I'll go back to my original suggestion, and recommend you try PrimoPDF. It's free and works in the same way that your current option works. It's never failed me, so could be worth a try, and may sort out all of your problems.
Edit: I've just tried it, setting up the images using the print dialogue like you're doing, and it seems to work fine... the gradients look good to me. PDF Output
I'm not sure I understand what you're after, what do you mean by 'I need it for imposition'? I understand that you're getting buggy results with Print to PDF, but if you can save to PDF okay, then why not just do that?
If on the other hand you can only save to CDR and need a PDF, then one quick alternative might be to have a go with PrimoPDF it fits into Windows as if it's another physical printer, but then lets you output to PDF.
I have very little experience with raster editors, so I cannot give an informed opinion on them.
I am aware, though, that there is a whole ecosystem of plug-ins for Photoshop, and some of them are not just for working with photographs.
As one example, look at some of the stuff from the Eye Candy plug-in:
Blender would be a good, and free choice.
The learning curve would be steep, especially for a complete beginner, but there are plenty of tutorials online to help. And, there's an active Reddit community /r/blenderhelp
Only reason: i have more experience with it, and stuff is faster to do it
it still has many annoying bugs when importing PDF and AI files, exact same behavior from the one I was using before (X6, year 2013). Probably because the engine underneath is exactly the same, the ancient ghostscript 8.64 (https://www.ghostscript.com/release_history.html - 12 years old!!!!! AGES old!!!!!)
i upgraded to 2019 and i did not chose the upgrade protection as for example one of my killer apps (finecut) get updates very slowly so for example now they published the update that let it work with 2018, with 2019 you can't install it yet. (anyone know a workaround?)
2019 will be my last one. If after 4 years i would need a newer version it still would be cheaper to buy a 700 euro license than 350+100 euro in 2019 + 300 euro in 2020-21-22. Plus the undeniable advantage that being a totally full version i can put the 2019 version in an older PC while having the 2022 version on the new one