Install Save Emails to Google Drive from Chrome Web Store.
Key features:
- Export all emails under a Gmail label — including its sub-labels — and save them in Google Drive while maintaining the original label hierarchy.
- Save emails in different formats: PDF, HTML, EML, TXT
- Merge multiple emails into one files (PDF, HTML, etc.)
- Merge emails body and attachment in one PDF or HTML
How to get started and troubleshoot this extension is explained in support note.