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.

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As a workaround, you can create a content approval flow to approve only PDF files and reject all other file extensions uploaded to a document library.

Here is a reference: Require approval of documents in SharePoint using Microsoft Flow.

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.

UPDATE:

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.