Email Archive File Formats Explained: OLM, PST, OST & MBOX

Email is not just communication. It is documentation. Contracts, approvals, invoices, support conversations, and legal evidence are all stored in inboxes. But emails are not stored as simple files. They are saved in special archive formats that bundle entire mailboxes together.

In this guide, we explain:

1. Why bundling and converting emails to PDF is useful

2. What OLM, PST, OST, and MBOX files are

3. How you can convert or merge them into PDF on PDFen

1. Why Bundle and Convert Emails to PDF?

Email programs do not store messages as loose files. Instead, they use container formats that hold entire mailboxes, including folders, message threads, metadata, and attachments.

That works well inside email software. It is less useful for sharing, archiving, or legal use.

Converting email archives to PDF solves several problems.

Long-Term Archiving

Email formats depend on specific software such as Outlook or Apple Mail. PDF is universal and can be opened years later without needing the original email client.

Legal and Compliance Use

PDF files are easier to submit as evidence or documentation. Emails remain structured, readable, and complete, including attachments.

Easy Sharing

You cannot send someone a PST or OLM file and expect them to open it. Almost everyone can open a PDF.

Structured Documentation

When converted properly, emails remain organized:

• By folder structure

• In chronological order within folders

• With attachments placed directly under each email

This turns a messy mailbox into a clear and navigable document.

2. The 4 Main Email Archive File Formats

These formats all bundle emails, but they come from different ecosystems.

OLM (Outlook for Mac Archive)

Used by: Microsoft Outlook for Mac

What it contains:

• Emails

• Folder structure

• Contacts, calendars, and notes

Advantages:

• Full mailbox export for Mac users

• Preserves folder organization

Compatibility:

• Designed for Outlook for Mac

• Not directly readable in Windows Outlook without conversion

--> Tip: Convert your OLM file to PST to enable opening it in Outlook

PST (Personal Storage Table)

Used by: Microsoft Outlook on Windows

What it contains:

• Emails

• Folders

• Contacts, calendar items, and tasks

Advantages:

• Standard Outlook archive format

• Supports large mailboxes

• Widely used in business environments

Compatibility:

• Native to Outlook for Windows

• Not easily opened without Outlook

OST (Offline Storage Table)

Used by: Outlook with Exchange, Microsoft 365, or IMAP accounts

What it contains:

• A synchronized offline copy of a mailbox

Advantages:

• Lets users work offline

• Automatically syncs with the mail server

Compatibility:

• Linked to a specific Outlook profile and account

• Not designed for direct sharing, so conversion is often required

MBOX (Mailbox File)

Used by: Apple Mail, Thunderbird, and many Linux mail clients

What it contains:

• A collection of emails stored in a single text-based file

Advantages:

• Open and widely supported format

• Used across many email systems

Compatibility:

• Supported by many email clients

• Still not easily readable without mail software

Summary

3. Converting or Merging These Files on PDFen

PDFen allows you to convert individual files or merge multiple email archives into one single PDF.

Supported Formats

You can upload:

• OLM

• PST

• OST

• MBOX

What Happens During Conversion

PDFen converts emails in a logical and readable way:

• Emails are grouped based on their original folder structure

• Inside each folder, emails are ordered chronologically

• Attachments are converted as well and placed directly under the email they belong to

This keeps the context of every message intact.

Convert a Single Email Archive to PDF

1. Upload your OLM, PST, OST, or MBOX file

2. PDFen processes the emails

3. You download a structured PDF version of the mailbox

Merge Multiple Email Archives into One PDF

You can also upload multiple email archive files and combine them into one unified PDF. This is useful when:

• Collecting emails from multiple people

• Combining archives from different periods

• Creating one complete case file

PDFen merges everything into a single structured document.

Final Thoughts

Email archive files like OLM, PST, OST, and MBOX are powerful, but not practical for sharing or documentation. Converting them to PDF turns complex mailbox data into a universal, structured, and long-term format.

Whether you need a single archive or a fully merged email history, PDFen makes the process straightforward while preserving the original structure of your emails. Want to know more about email conversion? Click here.