Why will this file not open, and what do I do about it?
How to work out what a file actually is from its extension, the free ways to open the awkward formats, and the file types you should not open at all.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 3 min
Short answer
Look at the letters after the final full stop — that is the file type, and it tells you what opens it. If nothing on the computer handles it, converting it or using a free viewer usually works. Never open an unexpected file ending in .exe, .msi, .scr or .js, whatever it claims to be.
Computers hide file extensions by default, which is why an unopenable file feels mysterious. Turning them on makes almost every case obvious in seconds.
Step by step
- Turn on file extensions.Both Windows and macOS have a setting to show the full filename including the extension. It is worth turning on permanently — it is also a genuine security measure, since hidden extensions are how a program is disguised as a document.
- Read the extension.The letters after the last full stop. Common ones: .docx word processing, .xlsx spreadsheet, .pdf document, .heic modern phone photograph, .zip compressed folder, .csv plain data.
- Try opening it with something you already have.Right-click and choose 'open with' to see what the computer suggests. Most office and image formats open in more than one program.
- Use a free web-based viewer for office formats.The free online versions of the major office suites open most word processing, spreadsheet and presentation files without buying anything.
- Extract compressed files rather than opening them.A .zip is a folder in a wrapper. Both systems open it with a right-click extract. Other formats like .rar and .7z need a free extraction tool.
- Convert phone photographs if they will not open elsewhere.Modern phones save in .heic, which older software cannot read. Phones have a setting to capture in the older, universally compatible format, or to convert on sharing.
- Refuse the dangerous ones.An unexpected .exe, .msi, .scr, .js, .bat or a document arriving with a demand to enable macros should be deleted. A genuine invoice is a PDF; a program pretending to be one is not.
- Ask the sender for a different format.For anything from an obscure or specialist program, asking for it as a PDF is faster than finding software for a format you will never see again.
Tips
- A file with no extension at all can often be identified by opening it in a plain text editor — the first line frequently names the format.
- Renaming the extension does not convert the file. It only changes which program tries to open it, which usually fails messily.
- For a file that arrives in an email you were not expecting, verify with the sender by another route before opening anything at all.
Common mistakes
- Downloading a program that promises to open any file — That category of search result is heavily populated by bundled junk and subscription traps. Free viewers from the format's own publisher are safer.
- Enabling macros because the document asks — Macros are program code inside a document, and the request is one of the most reliable indicators of a malicious file. No ordinary letter or invoice needs them.
If it doesn't work
File opens as gibberish text
Cause: The wrong program is opening it — Fix: Use 'open with' and choose a different program, or identify the format from the extension first.
Windows says it does not know how to open it
Cause: No program associated with that extension — Fix: Search the extension by name to identify the format, then use a free viewer for it. Do not accept the store's first suggestion uncritically.
Photos from a phone will not open on an older computer
Cause: The .heic format is newer than the software — Fix: Change the phone's camera format to the compatible option, or share the photos in a way that converts them automatically.
Document opens but is unreadable or asks for a password
Cause: It is protected, or it was corrupted in transfer — Fix: Ask the sender for the password or a fresh copy. A file damaged in transfer usually cannot be repaired.
Questions people ask
Is it safe to use a free online converter?
For an ordinary document, usually. Do not upload anything confidential — you are sending it to a company you know nothing about. For sensitive files, convert on your own computer.
Why does my computer hide extensions?
It was a tidiness decision made decades ago and it has caused problems ever since, because a file called invoice.pdf.exe appears as invoice.pdf. Turn them on.