Hi @Sk_agl, welcome to our forum!
I just had a look in an Xperia XA and I downloaded a user guide from our homepage.
I was able to open the PDF file with Drive PDF Viewer which is inbuilt in the Google Drive application, so there was no need to download a new PDF viewer from the Google Play store. If you don't like the look or how this specific application works, you can always have a look in the Google Play store for other PDF viewers, I'm sure you will find one that you like!
You can access the downloaded file(s) in the application called "Downloads" in your phone.
Let me know if that helps! 
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