I tried the module as per your suggestion and it does open the PDF properly.
For my use case, the client exports a ticket to be signed. When the signed ticket returns they scan in it and save as PDF, then attach it to the invoice and print/save it.
To attach the invoice I readFileAsBytes(), save it in the database, then display it as the second page in the Report Viewer in an Image Placeholder.
In this case I am unable to use the PDF Viewer module so I’m not sure how to handle this.