You're right, page viewer web part is not supported in SharePoint Online modern experience.

As a workaround you can use below web parts:

1. To show the file, use [File viewer web part](https://support.microsoft.com/en-us/office/use-the-file-viewer-web-part-e53a9602-e7b3-4fff-9126-de2f18d1900c)
2. To show the site pages, use [Embed web part](https://support.microsoft.com/en-us/office/add-content-to-your-page-using-the-embed-web-part-721f3b2f-437f-45ef-ac4e-df29dba74de8)