Try download a PDF file from WhatsApp or Telegram or HIKE or similar messenger app.
Then try opening this file ( by searching in MY FILES in your Android smart phone ) in your smart phone. For the first time it will ask which App ( software ) should use to open this file.
This happens when in your mobile phone more than one PDF related app are pre-installed. Like Google PDF viewer, Adobe Acrobat Reader, PDF reader or similar.
SO while first time opening some PDF file you chose Google PDF viewer, so from next time your mobile phone will keep opening such PDF files from Google PDF viewer only.
We all know that Adobe Acrobat reader is the best App for opening such files as it supports SHARE feature, make comments on PDF, highlight and markup , take photo of documents and SAVE AS PDF and so on.
If you want to change the default app for opening PDF, you need to go to settings – Apps – then open Google PDF viewer app. At the bottom click Set As default. Then a new dialog box opens where you need to click on clear defaults.
This process will make your smart phone ask you again , which App to use, while opening PDF file again. This time choose Adobe Acrobat reader.
Similarly by mistake if you have chosen default App as Microsoft word for word files saved in your Android smart phone, you may follow the above process and click on Clear defaults in the APP Microsoft word .
Then when you open any word file choose Google File as default app.
Do share your comments or related issues. These issues may look simple but may waste lot of time for the first time user.