Someone sends you a file. You double-click it and get “How do you want to open this file?”, an error, or a screen full of gibberish. The fix starts with knowing what kind of file it is.
Step 1: Find the file’s extension
The extension is the few letters after the last dot in a file’s name, like .pdf or .docx. It tells your computer which program should open the file.
Windows hides extensions by default. To show them:
- Windows 11: in File Explorer, click View → Show → File name extensions.
- Mac: in Finder, go to Settings → Advanced → Show all filename extensions.
Step 2: Match it to the right program
| Extension | What it is | Open it with |
|---|---|---|
| Document | Your web browser, or Adobe Acrobat Reader | |
| .docx, .xlsx, .pptx | Microsoft Office files | Office, the free Office web apps, Google Docs or LibreOffice |
| .odt, .ods | OpenDocument files | LibreOffice, or Word and Excel |
| .pages, .numbers, .key | Apple iWork files | A Mac, or free at iCloud.com on any computer |
| .zip | Compressed folder | Built in: right-click → Extract All (Windows) or double-click (Mac) |
| .rar, .7z | Compressed folder | 7-Zip (free) on Windows; recent Windows 11 versions can open these too |
| .heic | iPhone photo | Mac opens it natively; Windows needs the HEIF Image Extensions from the Microsoft Store |
| .mkv, .mov, .avi | Video | VLC media player (free) plays almost anything |
| .csv | Spreadsheet data | Excel, Numbers or Google Sheets |
| .eml, .msg | Saved email | Your email program, such as Outlook |
Not on the list? Search online for the extension (for example, “what opens .dwg files”) and download the suggested program only from its official website.
Step 3: Fix “opens in the wrong program”
If PDFs open in the wrong app, or photos open in something you don’t like:
- Windows: right-click the file → Open with → Choose another app, pick the program and click Always. Or go to Settings → Apps → Default apps.
- Mac: right-click the file → Get Info, choose a program under Open with, then click Change All.
Step 4: If it still won’t open
- Download it again. Files sometimes get corrupted or cut off during a download.
- Ask the sender to resend it, or to save it as a more common type such as PDF.
- Check the size. A file that’s 0 KB is empty or broken.
- Is it password protected? You’ll need the password from the sender.
- Update the program. Older versions of Office and other apps can’t always open newer files.
File types to be careful with
Some file types can run programs on your computer. Be very cautious if you receive these unexpectedly, especially by email:
- .exe, .msi, .bat, .cmd, .scr (programs and scripts)
- .js, .vbs, .ps1 (scripts)
- .iso, .img (disk images that can hide programs inside)
- .html, .htm attachments (often fake login pages)
- Office files that ask you to “Enable content” or “Enable macros”
Also watch for double extensions such as invoice.pdf.exe. With extensions hidden, that looks like invoice.pdf. It’s one more good reason to show file extensions.
If you weren’t expecting it, don’t open it. Check with the sender another way first.
The bottom line
Show file extensions, match the extension to the right program, and set your default apps so files open where you want. And treat unexpected programs, scripts and “enable macros” requests as a warning sign, not a file to open.
