Hello Jac,
I hope you are doing well. Per the description, I understand that you want to make Acrobat Reader Mobile your default PDF reader.
You might want to start first by clearing the default that is set on your device. Please try the below steps:
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Go to Settings.
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Go to Apps.
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Select the other PDF app, that always opens up automatically.
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Scroll down to "Launch By Default" or "Open by default".
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Tap "Clear Defaults" (if this button is enabled).
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The next time you tap a PDF document, the app chooser should be shown, allowing you to select the PDF Viewer.
- When clicking on Acrobat Reader, select the option that either says "Remember my choice" or "Always"
This should now change your default PDF viewer to Acrobat Reader Mobile.
Let me know if this helps.
Thanks,
Souvik.