Why Finder opens the wrong app for a file

Select the file, press Command and I, open the Open with section, choose the application you want, then click Change All. That sets the default for every file of that type. If it reverts, the cause is almost always a setting stored on the individual file rather than a failure of the one you just made.

The three step fix works for most people. The rest of this page is for when it does not.

The fix

Step three and step four do different things, which is where most of the confusion lives. Choosing an application applies to the selected file alone. Change All is the button that moves the default.

How macOS decides in the first place

Three things happen in order when you double click.

macOS looks for a per file override first. If the file carries one, that wins outright and nothing else is consulted.

Failing that, it works out the file's type. Not by looking inside it in the great majority of cases, but from the extension on the name.

Then it asks its own database which application owns that type. That application gets a message telling it to open the file.

That database is built from the applications themselves. Each one declares the types it can handle. The entry appears when the application is first run from Finder. Install something that claims a type broadly and it can become the owner of files you never intended it to touch.

Why it reverts

Four causes. The first explains most reports.

The file has its own override. Choosing an application in Get Info without clicking Change All writes a setting onto that file. Change All later moves the type default. This file ignores it, because the per file setting is checked first. One stubborn file among hundreds that behave is nearly always this.

Two copies of the application exist. An old version in Applications with a newer one somewhere else. Sometimes an installer that left both behind. The database picks by version rather than by what you chose.

A reinstall re registered it. Updating or reinstalling any application refreshes what it claims. A broad claim can take back a type you had reassigned.

The database itself is confused. Rarer than the first three and the one people jump to first.

Fixing one file rather than the type

Sometimes the override is what you want. A text file you would rather open in a code editor while every other text file opens normally is exactly the case Get Info handles without Change All.

Two quicker routes exist for a single file. Right click and use Open With to open it once in something else, which changes nothing permanently. Or right click, hold Option, then Open With becomes Always Open With, which sets the override in one action.

To undo an override, use Get Info and set the file back to the application that ought to own it. The setting is stored on the file rather than in any preference, which is why it travels with the file if you copy it somewhere else.

The two defaults nobody separates

Worth stating plainly, because the interface does not.

The third column is the useful one. A setting stored on the file travels with the file. Copy it to a colleague's Mac and it arrives carrying an instruction about which application should open it, which is a reasonable explanation for behaviour that otherwise makes no sense on a machine you have never configured.

When Change All will not stick at all

Work down this list rather than starting at the bottom.

Only after all four does the database itself become the suspect. The repair for that is to make macOS rebuild it by rescanning every installed application. The command sits at a very long path inside CoreServices and takes a minute to run, after which a restart puts everything back in order. Apple supplies it, so this is no hack. It is also rarely the actual problem.

The extension is doing more than you think

Because the type comes from the extension nearly every time, a few ordinary situations produce the wrong application and look like a bug.

A hidden second extension. Finder can hide extensions, so a file shown as report is really report.rtf. Something shown as archive is archive.zip.

A file renamed rather than converted. Changing an extension by hand changes what macOS thinks the file is without changing a single byte inside it. The application then opens it and complains.

An extension nothing claims. Files from a converter or an unusual application carry types no installed software has registered, so macOS falls back to whatever will take them.

Reading a name properly is a small skill of its own. It sits alongside reading the paths those files live at.

The short version

Get Info, choose the application, Change All. If one file still misbehaves it has its own setting, so fix that file directly. If everything misbehaves, look for a second copy of the application before blaming macOS.

The database is the last thing to suspect, not the first.