When you drag-and-drop an email from Microsoft Outlook it saves the email and any attachments as a .msg file. This file format is acceptable in most cases, but for records with retention periods of 10+ years, or if you have people in your office who do not use Outlook for email, converting emails to PDF is the preferred solution.
If you're looking to transfer emails to another person or office, speak with them first about what method and format (e.g., snapshot, shared folders, forwarding, PST, MSGs, PDFs, etc.) will work best. Please note: preserving metadata is often paramount in legal matters. In these cases, forwarding or converting emails may not be the best approach as the process can potentially modify or destroy part of an email's metadata.