If you use Universal viewer to display pdf in Omeka S v4 with the site param “UV v4”, you need to upgrade the module: the viewer used the mozilla cdn for the pdf part but the file is now removed (https://mozilla.github.io/pdf.js/build/pdf.js).
The new version of the module uses another cdn (see CDN fix for PDF.js by 404mike · Pull Request #940 · UniversalViewer/universalviewer · GitHub).
If you cannot install the new version of the module (3.6.8), just select uv v2 or v3 in the site params.