preview opens all PDF files but it is not set as default

I experienced the same problem and was glad to find your post.

After experimenting a little, I think I found the solution. Open up PDF Expert and then select Settings from the menu. When the Settings window pops up, I think you'll see Preview as the default PDF reader. (How bizarre is that!). When you changed this setting to PDF Reader, the problem should go away.

Are you sure you made it default for ALL pdf files?

When you control-click on a pdf file, and choose Open With, which application shows up as default?

Preview or PDF Expert?

Select one pdf file and press Command-I to open the Info window. Does it show Preview or PDF Expert as the application to open this file? If it says Preview, select PDF Expert, AND... click the "Change All" button.

Now check a different pdf file. Does it now say PDF Expert? Does it open with it?

A few things I can think of, that you may want to try:

1) Test in Safe Mode. Does the same problem occur?

2) Reset launch services. I could only find these instructions for an older OS - I am not sure if they still apply.

Enter these three commands (each is one single line, though the first two are very long) in order in Terminal:

2a)

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user

2b)

sudo /System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -seed -lint -r -f -v -dump -domain local -domain system -domain user -domain network

2c)

killall Dock

Well, downloaded PDF Expert to test and once I selected a PDF, opened "Get Info" and changed the default from Preview to PDF Expert and clicked the "Change All…" button, PDF Expert opens my PDF documents by default.

I noticed when installing a dialog opened which asks if you want to make PDF Expert the default. I chose not to enable it and did it manually instead.

Now I have to uninstall PDF Expert.

Welcome. Try booting into Safe Mode then restart normally. Sometimes that will fix anomalies.

How to use safe mode on your Mac - Apple Support