Files can arrive from an email, chat, download, removable drive or shared folder. A familiar extension does not guarantee safe contents, and a successful preview does not prove the file is harmless. The safest approach is to inspect a copy before using an application that may execute scripts, macros or other active content.
1. Verify the source before opening
Ask whether you expected this file, whether the sender and download domain are genuine, and whether an urgent request to enable editing or install a viewer seems suspicious. When a document arrives unexpectedly, confirm through a trusted, separate contact channel. Avoid opening an unknown attachment just because a preview service shows text.
2. Keep the original and inspect a copy
Make a separate local copy before attempting to rename, convert or repair the file. Do not run executables, scripts or installers from unknown sources. For file types such as .exe, .bat, .ps1, .js or Office files containing macros, use reputable endpoint security tools and organizational rules rather than treating a browser inspection as a security scan.
3. Check the actual file type
A filename can be misleading: invoice.pdf.exe is not necessarily a PDF. Use the file type detector and binary hex viewer to examine header bytes. A recognized header identifies a likely format, but cannot authenticate its source or validate every byte.
4. Compare a hash when one is provided
If a publisher gives you a SHA-256 value through an authenticated channel, calculate your copy using the SHA-256 checksum tool and compare the full value. Matching a legitimate publisher's digest can establish byte-for-byte identity with that published version; a hash copied from an untrusted message is not independent proof of safety.
5. Preview with the least-powerful tool
Plain-text, CSV, JSON and ZIP listings can be examined without executing them. For DOCX, XLSX, PPTX and EPUB, use text-first local previews and remember that visual layout or embedded features may be omitted. Do not enable macros or bypass security warnings to make a preview work.
6. Decide when you need a specialist viewer
Encrypted archives, DRM-protected books, complex spreadsheets and damaged files may require trusted, format-aware desktop software. If a file requests a password, obtain it from the legitimate owner; do not use unknown online cracking services or upload confidential material to arbitrary viewers.
7. Interpret the result carefully
A readable preview means only that some content was decoded. It does not certify that the file is malware-free, complete, authentic, or suitable for legal or financial reliance. See the troubleshooting workflow if the file still refuses to open.