If you're on an iPadOS beta version, you might find that when you import a PDF to the app, some images are not rendered correctly in Goodnotes, for example, they may be squashed or squeezed.
Unfortunately, this is a typical iPadOS beta bug that might occur to some specific PDFs and if you open the original PDF in the Files app or Mac Preview, you should see the same issue:
Whereas the image above should look like this:
Since Goodnotes relies on Apple's PDF engine, we'll need to wait for an official fix from Apple in an official iPadOS software update. Meanwhile, you can help bring this to Apple's attention, and/or downgrade to an official iPadOS version.