Double-click. Notepad opens, showing what looks like jumbled code. '%PDF-1.5' at the top, then binary noise. That's a PDF file content but opened in the wrong app.
What's actually going wrong
File association wrong. Your OS thinks .pdf files should open in Notepad or a text editor. Extension stripped or mangled. The file is .pdf.txt or has no extension at all. Default app uninstalled. Your PDF reader was removed and the OS doesn't know what to use.
The file itself is fine. The OS just doesn't know how to handle it.
The quick fix
Right-click the file. Choose 'Open with' > pick a PDF reader (Chrome, Edge, Acrobat, Preview). Tick 'Always use this app' to make it the default for next time.
If the file extension is wrong (e.g., .pdf.txt), rename it to remove the .txt suffix. The OS then treats it as a PDF.
If that didn't work
If no PDF reader is installed, install one. Chrome and Edge both open PDFs natively without additional software.
For files with truly no extension, add .pdf to the filename. The OS should immediately recognise it. If your file manager hides extensions by default, enable showing them temporarily to confirm.
Prevent it next time
Keep at least one PDF reader installed. Don't strip file extensions. And set Chrome or your preferred reader as the default PDF handler.
FAQ
Why is my PDF showing as a text file in the first place?
PDFs are technically text-based files with binary content. Opened in a text editor, you see the structure but not the rendered content. Open with a PDF reader instead.
How do I change the default PDF app on Mac?
Right-click the file > Get Info > Open with > select PDF app > Change All. Future PDFs open with that app by default.
How do I change the default PDF app on Windows?
Settings > Apps > Default apps > .pdf > select your preferred reader. Every PDF opens with that app from then on.
Can I open a PDF without any reader?
Browsers (Chrome, Edge, Firefox, Safari) all open PDFs without additional software. Drag the file into any browser window — it opens.
Once your PDF opens properly, drop it into Flint for editing, compression, or conversion.