How to Open Unknown Files
Start with the full filename
Find the letters after the final dot in the filename. A file called report-final.PDF has the extension PDF, while invoice.v2.docx has the extension DOCX. Write the extension down exactly as it appears, because case does not matter but punctuation can.
Search that extension in this database instead of guessing from the filename label. Filenames can be renamed freely, so "invoice.pdf" may not contain a real PDF at all.
Check the source and context
Ask where the file came from and why it should exist. A PDF attached to a booking confirmation after you made a booking is more contextually plausible than an unexpected PDF from a stranger.
If the file arrived by email, open the sender address and look for signs of impersonation. If you are not sure, contact the sender through a separately verified channel rather than replying to the message.
Choose software for the actual format
Use the compatible software listed on the extension detail page. For example, a .HEIC photo may need Apple Photos or a Windows codec, while a .MKV video plays well in VLC.
Avoid renaming the file to force another program to open it. Renaming .png to .jpg changes only the label, not the image data.
Example: opening an unknown .CRDOWNLOAD file
A .CRDOWNLOAD file appears in your Downloads folder after Chrome pauses or interrupts a download. The extension means the file is incomplete, not that it is a new video format.
The correct next step is to resume or restart the download in Chrome. Renaming the file to .MP4 will not finish the download and can create a broken file.