When connecting PowerApps to SharePoint, the built-in connector shows a list of Lists in the target site. However, you can type in the name of the document library in the text box "Enter a custom list name"
Once you enter the library name, the app is built as usual. You can then customize the BrowseGallery to include a button that uses the Launch() function to open the document.
Alternatively, you can customize a card on the detail form. (That link discusses using an image control, but the OnSelect action on a button can also reference the DataCardValue property.)