Read the whole filename
Systems hide extensions by default. Look at what follows the last dot: invoice.pdf.exe is a program, not a PDF.
THE 60-SECOND CHECK
Not sure that file is safe?
The extension is only a clue. Check the full name, where it came from, what it actually is, and whether anything looks off — then decide.
✓
Ask who sent it — and whether you were expecting it
The usual case is a hijacked account sending an attachment nobody asked for.
Match the format to the story
A quote does not arrive as a script or an installer. When those two disagree, the story is the part that is wrong.
Open it in a viewer before an editor
A viewer only displays the contents. Editors and office suites run macros and fetch remote content.
Stop when your system, browser, or antivirus objects
The warning may be wrong, but it is worth thirty seconds spent confirming where the file came from.