theThe easiest way is to use the Google Drive website on your computer (not on the phone). Right click the PDF file, then Open With, and choose Docs. It will automatically convert the whole PDF file to a Doc file. Unfortunately, this option isn't available in the app.
It is possible to do it in the mobile Chrome browser, but you have to go to drive.google.com, and then tap Menu and toggle on Desktop Site. Then long-press the PDF file there, select Open With, and then choose Docs.