Method 3. Open LibreOffice itself rather than Writer, Calc or other module/program/app. You might have seen this if you close all documents without exiting from LibreOffice. You get a kinda launch page or splash screen (actually the real splash screen is the banner you get when opening LibreOffice, just before it's properly open). This has a list of recently opened documents in a panel to the left and a grey empty space to the right that contains a white dialogue-box which contains icons of each different app/program/module. Drag the file into the grey-space there.