If I start the other app and open a PDF file, it's fine. I click on it and there is a ' Clear Defaults' button which is faded out and unclickable. The only option of any consequence here is ' Open by Default' which says ' No defaults set. I go to the setup -> General -> Apps -> Amazon Kindle. Now I want to use another app to open it, but AMAZON KINDLE refuses to relinquish control. But I don't like it for a number of reasons but mostly, It is slow. When I went to open up a PDF file, I was offered the option to use Kindle, since that was all that I had at the time and it was time-critical information so I just used Kindle. Last November I got an Aristo with Android 7.0 installed. I used to have a PDF reader on my old phone. If it doesn't, might it be that the app you are using to open the PDF is itself choosing to open it using Kindle, rather than go through the OS? Alternatively, adding a new app which can also handle the intent should prompt the system to ask again next time you try to open a PDF, even if a default is set. With no default set it should give a list of all apps capable of handling an intent (i.e. I ask these questions because of the way Android handles 'intents'.

How do you open the PDFs? What other PDF-readers do you have installed? What happens if you install a new one (or uninstall and reinstall an old one)? So that's what we need to work out, why your Kindle app is automatically opening PDFs without the default being set.

So it's not that the (5 year old) solution above is incorrect, but that your problem seems to have a different pathology, and hence probably a different cause. * open a fourth pdf - it asks which app to use. * go to Settings > Apps, select Kindle, there are defaults set. * open a third pdf - it doesn't ask, just uses Kindle * open a pdf - it asks what app to use, I select Kindle So can you tell us a bit more about your device or setup? Because I just tested this, and for me it worked as expected:

Samsung change default app for pdf