Thanks for contacting Syncfusion support.
We were tried to reproduce the issue based on the previous update in the forum and sample code, but we are unable to reproduce the reported “PdfViewer throws when on Unload” in our side. Based on the Forum details we suspect that you are using the Syncfusion product version 15.1 or 16.4. On analyzed further, we have found that we have resolved some issues in our recent days related to the same and the reported issue might be resolved in our latest version of PDF Viewer.
So, we request you to try our latest version of PDF Viewer and let us know whether the issue is resolved. If you still face the issue, please provide us the following details that will be helpful for us to analyze more and provide you a better solution.
1. Replication steps to reproduce the issue
2. Syncfusion Product version.
3. .Net Framework / .Net Core version.
4. Operating System.