Try one file before changing every file
Use the file manager's Open with option, or open the intended application first and select the file from there. Work on a copy if you are testing unfamiliar software. Confirm that the contents, layout and any required editing functions are present. Being able to display a document is not the same as preserving all its features when saving.
If the application cannot open the file, check its supported formats and version requirements. Renaming the extension does not convert the underlying contents. Changing an image filename to end in PDF, for example, does not create a PDF document. Use an appropriate export or conversion process when a different format is genuinely needed.
Set a default for the right type
Use the operating system's default-application settings or its documented file-information controls. Select the specific extension or type where possible. Do not accept a broad “make default for everything” prompt unless that matches your intention. An application installed for one task may otherwise take over unrelated file types.
Test a second ordinary file of the same type, then check a different format that should remain unchanged. If the association resets repeatedly, investigate application updates or managed-device policies rather than repeatedly editing system internals. Work devices may deliberately enforce particular defaults.
Keep source files intact
When saving from a different program, read any compatibility warning. Use Save As or export to a separate destination until you have verified the result. This matters for spreadsheets with formulas, documents with complex layout and images with editable layers.
If you only need to read a file occasionally, opening it manually in another application may be enough. A permanent default is useful when the choice fits most future files of that type. Keep that distinction in mind before changing the behaviour for every document on the computer.
