A document ends in .pdf, a spreadsheet in .xlsx, and an image in .jpg. These extensions help the operating system suggest an application. They do not guarantee either the actual contents or the safety of the file. Making extensions visible and treating them deliberately prevents common mistakes when opening, sharing, or converting material.
Association is not conversion
If a PDF opens in the wrong application, changing the default application may be all that is needed. The document does not need a different extension. Conversely, renaming photo.jpg to photo.png does not create a PNG image. Conversion reads the content and writes it in another format, usually through Save As or Export.
Keep the source before conversion. Some formats cannot retain layers, formulas, comments, or other features. Check the result against the properties you actually need. Successfully opening the new file does not establish that every important feature survived.
Inspect the complete name
Show filename extensions in the file manager. Otherwise, names containing multiple dots can be misleading. A file with invoice in its visible name is not necessarily a PDF. For unexpected attachments, check the reported file type and the context of the message too.
A familiar icon is not a safety certificate. Do not run an unfamiliar executable merely to discover what it contains. If an expected document arrives in a surprising format, verify it with the sender through a known channel. An unusual format may be legitimate, but it needs an understandable explanation.
Export for the recipient's task
A widely supported output may be enough for reading, while further editing requires the source. Ask what application the recipient uses when specialized features matter. Exported text can be readable even after losing the original comments or document structure.
For repeated handoffs, run a small trial: create the file, export it, open it in the destination application, and perform a typical task. Record the suitable export option. The extension then becomes a useful clue in a known process rather than a reason for repeated experiments with arbitrary renaming.