If you are looking for a solution to download ProtonMail emails with attachments then I would recommend you to use
Mail Backup Tool on your system. The software can download all emails in a single attempt. Apart from this, it also provides advanced filter options to save selected ProtonMail emails. it offers various saving options to
backup ProtonMail emails in multiple formats such as PST, PDF, MBOX, EML, etc.
Steps to Download ProtonMail Emails with Attachments
1. Run Mail Backup tool on your system.
2. Select ProtonMail and enter its login details to sign in.
3. Choose mailbox folders.
4. Select a saving option from given list.
5. Browse target location and click on Backup.
The software will start downloading ProtonMail emails in your desired file format.