Does anyone know if there is any alternative for Page Viewer web part in SharePoint modern page online?
I have found it in classic page but I need in modern page.
You're right, page viewer web part is not supported in SharePoint Online modern experience.
As a workaround you can use below web parts:
Update from comments:
If you want to develop your own web part, you can use the SharePoint Framework. Check below documentations for more information:
If you don't like embed web part or File viewer web part, you can try to use custom web parts like modern script editor to directly insert HTML code.