To generate QR codes, you will need the link to a document or webpage. If linking to a Google doc or slide presentation, check your sharing settings.
QR Code Monkey - free QR code generator for links, can download the QR code as a PNG file (no sign in required)
QR Code Generator - free QR code generator (can accommodate links to sites/documents as well as text and other items (no sign in required)
To view QR codes - download a QR code reader for your phone from the Play Store or App Store