I'm using SharePoint Online modernUI.
In our site library, we want to restrict users to uploading only PDF files.
Do you have any ideas to verify file extention when uploading?
I would really appreciate it if you could deal with this matter.
This feature is not yet supported in SharePoint Online.
Please see the SharePoint UserVoice post here: Custom restrictions of file types that user can upload to a document library.
As a workaround, you can create a flow to approve only PDF files and reject all other file extensions uploaded to a document library.
Here is a sample flow for your reference:
Turn on Content Approval for the document library. The flow will delete new uploaded documents that are not PDF files and approve PDF files automatically.
Here is a reference: Require approval of documents in SharePoint using Microsoft Flow.