I don't see anything but two things:
1. How are you creating the PDF?
2. How are your viewing the PDF?
Thin white lines are usually a result of flattening transparency in a PDF and it's called stitching.
If it is stitching, these won't print but can be annoying on screen, sometimes hard to convince a client, you can make these go away in Acrobat by going to Preferences / Page Display / untick smooth line art and smooth images boxes
You’re going to walk a client through that?
Why would you flatten a PDF, anyway? Export it properly with live transparency and avoid the issue.
I know its not ideal, but we have to supply all pdf to PDFX1a standards for various reasons, our clients do understand and are quite happy to go through these prefs changes - for peace of mind if nothing else.