I've had this PDF problem on my Mi Pocophone F3 for some time but didn't really bother to fix it. Everytime when I open a pdf document on the phone, CamScan pdf program opens the PDF. The problem when CamScan opens the pdf it thinks I want to convert the pdf into other formats (word doc, powerpoint etc) so I'm not able to read the pdf on screen. CamScan is an app that scans any documents from the phone's camera and then converts it to scanned pdf copies on phone.
Here's how to reset (or remove) the default program that opens pdf.
Open the settings menu (swipe down and press the gear symbol), type in "app" keyword is search. Select "Apps", Select "Manage Apps"
Select the CamScanner program (the wrong default program) then at the bottom (cannot see from screen) there is a "clear defaults" . Press "Clear Defaults" and the default pdf reader opening behavior will be reset.
On the next launch of a PDF document, you will be prompted to select the default application to open pdf. Now you can select the correct application to open pdf or even set the correct default application.
The steps outlined are specific only to Mi Pocofone F3, I believe for other Andorid models the steps would be slightly different but more or less similar.