I too am confused by this message.
Hmm, I just updated to WP 5.6, and I think it worked before that!!!!!
I have https://www.centuryhouseassociation.com/?page_id=8446
While editing the page, the Preview works fine, and so does View Page after I Update. But viewing the page in a separate I get the error message.
I installed the plugin, and used its WP block to embed a PDF from my WP’s media library. At no time did I make any choices or overrides to where things go.
I see that “both the viewer and PDF have to be on the same domain” but how are they not?
In “Edit as HTML” I see
<div class=”wp-block-pdfjsblock-pdfjs-embed pdfjs-wrapper”>[pdfjs-viewer viewer_width=0 viewer_height=800 url=https://www.centuryhouseassociation.com/wp-content/uploads/2020-12-Clarion-Web.pdf download=true print=true fullscreen=false fullscreen_target=false fullscreen_text=”View%20Fullscreen” zoom=auto search_term=”” ]</div>
The URL of the PDF is correct. How can I tell where the plugin is, and surely I don’t have to override the WP defaults on that???