A PDF is outside Android Reading mode's supported content types. A document can contain plenty of readable text and still be the wrong format for this feature. Check what you opened before reinstalling anything or changing the shortcut.
Identify the content, not just its title
Google's Reading mode help describes support for long articles on Android phones running version 9 or later. It excludes PDFs, email, chat messages and social feeds. Display and audio options do not remove those format limits.
For a hypothetical comparison, imagine three items all titled “Visitor instructions”: a normal web article, a downloaded PDF and a chat message. The matching title does not make them interchangeable. Write the format beside each item: web article, PDF document or conversation. Only the first is a suitable candidate for this Reading mode test; that does not guarantee every web page will work.
Try one public article as a control
Use a public, non-private article with a clear heading and several paragraphs. Note its heading and the opening words of its final paragraph. Open it with the Reading mode shortcut you have configured. Compare those two points with the original page, then check the paragraphs between them.
If the article opens but the PDF does not, record: “The shortcut opened the web article; the PDF is an unsupported format.” That result gives no reason to reinstall the feature merely to make the PDF work. If the article also fails, consult the official setup instructions and check the shortcut separately. Two failed attempts do not establish that both have the same cause.
Choose the right reading route
Keep using a trusted PDF viewer for the PDF itself. For a long web article, Reading mode may provide the reading presentation you need, but compare essential material with the original. Hearing a passage aloud does not verify its claims or guarantee that every image and table was included.
Do not install an unknown APK to force a supported-format exception. Our general screen-change checklist offers related reading context; the Google instructions above remain the feature reference.