This following one statement in the Document property on a PDF Viewer control will dynamically convert many document types to PDF you can display in your apps such as Word documents, PowerPoint presentations, HTML files. Loads more. For PDF files I don't think this is as consistent as the alternate technique below is.
Substitute( Gallery1.Selected.Thumbnail, "transform/thumbnail", "transform/pdf" )
There is a more consistent way of doing it with PDF documents using the Graph APIs which surface a "@content.downloadUrl" property for any file, but this property in particular works 100% of the time, and once again you only need to set the Document property on the PDF Viewer to this field using the PA_Graph_DriveItems Flow I've shared on a really long blogs that speaks to loads more things you equally do using those APIs.
Rather download the simplified version of demo app I've shared on that blog, and all the other Flows if you want to:
https://youtu.be/poKsDNYm9Jw
https://masteroffice365.com/powerapps/9/#DownloadSolution