To view existing Goodnotes files on a non-Apple device, you have several options.
Share documents on the web
The quickest option is to share certain documents for immediate access in any browser. See Share documents between iOS, Android, and Windows devices
Use Automatic Backup
Another option is to enable Auto Backup in PDF format with a supported cloud service such as Dropbox, Google Drive, or OneDrive. These PDFs can be found in the auto-backup cloud storage and viewed on any platform.
Export documents in PDF or Image format
Alternatively, you can export your Goodnotes documents to PDFs or Images, which can be viewed on any platform. It is important to choose the PDF format for export because if you send your files in .goodnotes format, your non-Apple device will not be able to read them.