Changing which app opens which file type in ChromeOS isn’t easy to find and there are caveats.
For most file types, the default is good enough, but for others, you may want to change the app that opens the file type because you like something better. For example, the default app for opening PDF files doesn’t currently support markups, signatures, or bookmarks. We recently recommended XODO for these additional features, but you still have to run XODO first and then open the file you want to edit from within the app. Opening a PDF in the file system will still open the file in the system’s default app.
But, you can change that.
First off, you can’t change the default app to a web app. It has to be an app installed to run locally. So, in the case for XODO, you have to install the Android app from the Google Play Store. The ChromeOS extension or the XODO web app won’t work.
You can find the Android app here: Xodo PDF Reader & Editor
In the case of XODO, once you have the Android app installed, you’ll need to open it once so that XODO will prompt you to allow it access to the local file system. Once you give it access, XODO will be added to the list of applicable default apps.
Now, go into the File system, locate a PDF file and highlight it.
Then, choose dropdown arrow next to “Open” at the top of the Files app and choose “Change Default.”
Change the default to Xodo.
Using this method you can alter the default app for just about any file type.