If you have imported a PDF and you find that some images are not shown correctly in GoodNotes, for example, they may be squashed or squeezed, sadly this is a limitation of iPadOS 15 at present.
Unfortunately, this is an iPadOS 15 bug that occurs to some specific PDFs and if you open the original PDF in the Files app, you'll see the same issue:
Whereas the image above should look like this:
Since GoodNotes relies on iPadOS PDF engine, we'll need to wait for an official fix from Apple.
For workarounds, please see this related issue: