For users of iOS13 the share dialog has changed how it works after updating from iOS12:
Tap the icon for edit in-place or select it from the context menu.
Swipe all the way to the right on the set of icons in the top of the dialog until you get to the 3 dots (more) select this option.
Now you will be shown a list of apps you can set as favourites, first tap Edit.
Scroll down to the app "Open in PDF Viewer" and tap the green circle this will add the option to my favourites.I have used PDF Viewer in this example you can do the same for Word or Powerpoint
Once done tap done. Then tap Done again.
Now "Open in PDF Viewer" is in the top set of Favourite icons, you can re-order these using the previous screen, simply drag the options up and down.
If you still help please Contact Support and we will be happy to help.
For Edit In-Place to work correctly the Files App Integration must be enabled, follow our guide here.