What is the ONEPKG file format?
A .onepkg file is a Microsoft OneNote Package - a single-file container used to export, share, or back up an entire OneNote notebook. Rather than sending a folder full of individual section files, OneNote bundles everything into one portable package.
Under the hood, .onepkg is a Microsoft Cabinet archive file. It opens with the standard MSCF magic bytes (4D 53 43 46) at offset 0, which any CAB-aware tool recognizes. Inside the archive you will find all of the notebook's .one section files alongside the .onetoc2 table-of-contents file that records how sections are organized.
To open a .onepkg, double-click it in Windows or choose File > Open in Microsoft OneNote. OneNote extracts the CAB contents and reconstructs a normal notebook folder in your notebooks directory. The package itself is not the working format - it is strictly a transport and backup mechanism.
Utilities such as 7-Zip can inspect or extract the raw contents without OneNote, since the outer wrapper is a standard CAB archive. The section files inside remain in the proprietary MS-ONESTORE binary format and require OneNote or a compatible viewer to read meaningfully.
The format debuted in OneNote 2007 as the "Send as Package" feature and has remained structurally unchanged since. For cloud-connected workflows, OneDrive sync has largely replaced .onepkg as the preferred way to move notebooks between machines, though the format remains useful for offline backup and migration.
Security & safety
RISK: LOWA .onepkg is note data packaged in a CAB archive, not an executable, so opening one in OneNote is safe. Indirect risks: (1) notes can EMBED file attachments, so a package from an untrusted sender could carry a malicious attachment you launch from inside the notes - let antivirus scan attachments; (2) OneNote attachments were abused in 2023-era phishing, so be cautious with packages from unknown senders and keep OneNote updated. Don't upload confidential notes to third-party online OneNote viewers.
Format details
in a nutshellPrograms that open ONEPKG files
Windows3 apps
7-Zip
Open-source Because .onepkg is a CAB archive, 7-Zip can open it to extract the inner .one/.onetoc2 files (you still need OneNote to read the notes). 
Microsoft OneNote (desktop app)
Free Double-click the .onepkg (or File > Open), choose a location to unpack; OneNote restores it as a normal notebook. The free OneNote desktop app is the primary tool.
Free OneNote viewer (e.g. BitRecover)
Free Open and preview notes from a .onepkg/.one without OneNote installed (read-only recovery/preview). 
macOS1 app
OneNote for Mac
Free Install the free OneNote; the cleanest path is to open the .onepkg once in Windows OneNote and sync via OneDrive, since the Mac app works through OneDrive notebooks. Technical details
deep spec| Container format | Microsoft Cabinet (CAB) archive - the outer wrapper is a fully standards-compliant `.cab` file |
| Magic bytes | `4D 53 43 46 00 00 00 00` at offset 0 - the `MSCF` CAB signature followed by four reserved zero bytes |
| Encoding | Binary, little-endian byte order throughout |
| MIME type | `application/msonenote` (primary); also seen as `application/onenote` and `application/vnd.ms-cab-compressed` |
| Compression | MSZIP or LZX compression applied per CAB folder entry, as specified in the Cabinet format |
| Internal contents | All `.one` section files of the notebook plus one `.onetoc2` table-of-contents file packed as CAB entries |
| Internal section format | MS-ONESTORE (Microsoft's binary OneNote section format), version tied to OneNote 2007 and later |
| Integrity checking | CAB per-folder checksums on archive entries; no additional application-level digital signature or hash |
| Encryption | None at the package level; individual `.one` section files inside may carry their own OneNote password protection |
| Typical file size | Tens of KB for simple notebooks up to several hundred MB for notebooks with heavy media attachments |
| On open behavior | OneNote unpacks the CAB automatically into a normal notebook folder in the user's notebooks directory; the `.onepkg` is not edited in place |
| Platform support | Windows (primary, full read/write via OneNote desktop); macOS (import via OneNote for Mac) |
| Introduced in | Microsoft OneNote 2007, via the "Send/Save as OneNote Package" export feature |
| Primary use case | Offline transport and backup of a complete multi-section notebook as one file; largely superseded by OneDrive cloud sync for cross-device sharing |
| Released | OneNote 2007 (Send/Save as 'OneNote Package') |
| Latest version | Tied to current OneNote desktop app file format |
| Open standard | Yes · royalty-free |
| Specification | learn.microsoft.com |
ONEPKG conversions
Community Q&A
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