As far as I know, heaps of solutions are available on the web, which provides a secure way to export Office 365 mailbox to PST format.
Read this article: https://technologycounter.com/software-detail/shoviv-office-365-to-pst-converter-software
This post will help you to know about a professional solution that works to export Office 365 mailboxes into PST format.
Hope it works
MailsDaddy Office 365 Backup Tool is the best solution to export the Exchange Online mailboxes into On-Premise formats including EML, MBOX, PST, and MSG file. The users make use of this affordable and professional software to export Office 365 mailbox to PST without any technical issue. The Office 365 Backup Software is easy to use and implement the process of migrating O365 to PST format. It known to backup Exchange Online mailbox without damaging the single content of the PST data.
Looking for a secure way to export Office 365 mailbox to PST file format, then try one of the most secured and reliable Cigati Office 365 Backup & Restore tool. It can transfer all your Office 365 backup emails into PST, EML, EMLX, OST, HTML, TXT, MBOX, MSG file formats. This tool can backup and restore all your metadata of the mailbox such as emails, contacts, journals, calendars, etc. in multiple formats.