Start with the right checks.
An attachment can fail because it is blocked, incomplete, unsupported or simply needs the right application. The error text matters.
What to do next.
Confirm that the message and attachment were expected.
Read the file extension and exact warning without running the file.
For a trusted attachment, try the preview or supported save option in your Outlook version.
Open the saved file only with the intended, updated application and an approved location.
Check the result.
Confirm that the complete document opens and that any edits are saved in the place you intended. Previewing a document is different from saving your changes.
Stop and get help if…
The file asks for a password from an unfamiliar page, requests macros or differs from what the sender described. Stop and verify before trying another download tool.