To Import PST to Office365 Archive Mailbox manually, you can use the Network Upload or Drive Shipping methods provided by Microsoft. The Network Upload method involves uploading the PST files to Azure Storage and then mapping them to user mailboxes via the Import Service in Microsoft Purview. The Drive Shipping method requires copying PST files to an encrypted hard drive and sending them to Microsoft for upload. However, these manual methods are complex, time-consuming, and require technical expertise. Challenges include large PST file sizes, the risk of data corruption, mapping errors, and potential downtime. 

To simplify the process, you can use the MCD PST to Office 365 Migration Tool, which provides a seamless and automated way to import PST files. This tool ensures secure data transfer without size limitations, maintains data integrity, and allows selective migration of emails, contacts, calendars, and attachments. It also offers advanced features like incremental migration, eliminating duplicate imports. With an intuitive interface, even non-technical users can easily migrate PST to Office 365 Archive Mailbox without complications, making it a reliable and efficient solution.