Windows Explorer itself doesn't display thumbnails for PDF files as far as I know. I looked earlier and if it does, it requires settings changed in order to do so.
Windows Explorer will allow 3rd party applications to install handlers in order to generate thumbnails to be shown in it.
I looked up the orange icon and found it is Foxit. Next time, the more info you include, the better. Foxit has a Shell folder within where it's program files are and there is a DLL file which generates the thumbnails. I installed Foxit on my test system and the thumbnails showed up correctly for me.
If you have any other PDF software installed, it may be the culprit. Or it could be a buggy version of Foxit. Or possibly a video card issue.
A couple of things to try:
Try the most recent version of Foxit if you are not already using it.
Try a clean install of your video card drivers using the instructions here. Try both the latest version and a slightly older version of your video card driver.
Thanks for replying! Well, it's my first time posting here so something had to be lacking
I'll be more thorough next time.
So, I'm already using the latest version of Foxit Reader, installed Adobe Reader and the results are the same.
Uninstalled Foxit Reader and Adobe Reader and it defaulted back to Edge.
I think that the PDF files are the likely culprits, because some of them don't display those lines while others do.
My NVIDIA RTX 3070 still has drivers from May because the releases have been so unstable that I can't constantly replace the drivers.
But it's the first time I'm experiencing this... I don't know. I'll look for a solution later.