Hi Matthew,
We were unable to reproduce the issue with provided information. Please let us know how you are setting the background image to PdfViewer and kindly share the following details to analyze more on this issue and assist you with the better solution.
1.Simple sample to reproduce the issue.
2.Replication video/procedure to reproduce the issue.
3.PDF Document
4.Platform
5.SfPdfViewer version.
6.Xamarin Forms version.
Regards,
Sarath Kumar.