Note: Before you can open a document in Lumin from Google Drive, you must install the Lumin Google extension. Find out how here.
Go to your Google Drive.
Right-click the document you wish to open.
Hover over the Open with menu > click on Lumin PDF - Edit or Sign Documents.
Your document will now load and open in the Lumin editor, where you can view and make edits to the document.
TIP: Save any changes you make in Lumin back to Google Drive, find out how here.