If you have a huge number of emails and want to download or back up them, I will recommend the Cigati Gmail Backup tool. This tool has a huge number of features and will allow you to download all emails in PDF or other formats. It will also help you if you want to restore these emails on other email clients.
Download and Run the Cigati Gmail Backup Tool on your system.
Enter the credentials of your Gmail account that you want to download.
Now, choose the Emails to backup.
Tick the Gmail Folder you want to Download and press the Next button.
Choose the PDF File Format.
Next, Choose a location where you want to Save files.
Hit the Convert button.
At last, click on the Download Report button. Your files are downloaded in PDF format.