How to resume reading document opened in google pdf viewer
It appears that Google Chrome first have the file downloaded through Downloads app and then have it opened in the app chosen by user. Well in this case, you can just drag the Downloads app icon from your launcher's app drawer onto your home screen. Tap the icon and behold all your downloaded files available to you.

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You can also try beeshyams' approach of using a third party file manager app, such as ES File Explorer or Amaze File Manager. In both the apps, you can long press on the Download directory and under option choose Add Shortcut or Add to desktop. The folder would now be found on a new home screen as an icon.