What is a PUB File?Proprietary

📖 What is a PUB File?

A PUB file is a document created by Microsoft Publisher, the desktop publishing application Microsoft shipped from 1991. It holds page layout work such as brochures, newsletters, flyers, posters, catalogues, certificates and business cards. This is different from a .pub public-key file created by SSH tools such as ssh-keygen or PuTTY, which shares the extension but is an unrelated plain-text cryptographic key.

The distinction that matters is how a PUB file thinks about a page. A word processing file such as DOCX describes text flowing down a page, with images anchored into that flow. A PUB file does the opposite. It stores a set of independent objects, each pinned to an absolute coordinate on a specific page. A headline is not "the first paragraph" but a text frame at a fixed position with fixed dimensions. This is what makes Publisher good at layout and what makes PUB files awkward to convert into anything that expects flowing text.

⚡ Quick Facts

Neither MIME type is registered with IANA, which is consistent with a format that was never formally specified.

🔬 Inside a PUB File

From Publisher 2.0 onward, a .pub file is not one continuous block of data. It is a Compound File Binary Format container, sometimes called OLE2 or a structured storage file. The easiest way to picture it is as a miniature file system packed inside a single file, with its own directories and its own separate data streams.

This container is not unusual in itself. Microsoft used the same wrapper for legacy .doc and .xls files, and the container format is publicly documented. What sits inside the container is where Publisher diverges. The way it encodes pages, objects, their coordinates and the relationships between them is entirely proprietary.

Stream names vary across Publisher generations and this diagram is illustrative rather than exhaustive. The practical point is the separation. Metadata sits in documented, readable streams, while the layout itself sits in an undocumented one.

🔖 PUB File Signatures

Because the .pub extension has covered several generations of internal format, file identification relies on signatures rather than the extension alone. Publisher 1.0 wrote a flat binary file with its own magic header. From version 2.0 the file became a CFBF container, and the signature moved inside, onto the Contents stream.

Signature values as catalogued by the Archive Team file format wiki. Because no official specification exists, community documentation of this kind is the primary public reference for the format.

🔒 Why the Format Stayed Closed

In February 2008, Microsoft released documentation for its core binary Office formats, Word, Excel and PowerPoint, under the Open Specification Promise. Third-party developers could finally implement them properly. Publisher was not included.

The omission was deliberate rather than an oversight. When a developer asked directly on Microsoft's own binary file format forum whether the Publisher specification would be published, a Microsoft escalation engineer replied that there were no plans to document the .pub file format. That position never changed over the following years.

No Specification

Word, Excel and PowerPoint were documented in 2008. Publisher was left out, and Microsoft confirmed it had no plans to change that.

Reverse Engineering Only

Every non-Microsoft reader of PUB files was built by studying real files and inferring the rules, not by following documentation.

libmspub

The open-source library behind LibreOffice's PUB support. Impressive work, but partial coverage, which is why complex files fail.

Version Drift

Each Publisher generation changed internal structure. A parser tuned for 2016 files may not read a 1997 file at all.

No Ecosystem

Closed formats attract few implementers. PUB never developed the broad third-party support that DOC and XLS eventually gained.

Preservation Risk

A format readable by exactly one application, from one vendor, is a textbook digital preservation problem.

⚔️ PUB vs Other Layout Formats

PUB is one of several page layout formats. Comparing them on openness and portability explains why PUB is the difficult one:

See also the sibling references for PDF and DOCX.

💻 Software Support for PUB

Support for the PUB format is narrow and uneven. This table records what each application can actually do with a .pub file, rather than what it advertises:

To inspect a .pub file on Windows, the free Univik PUB Viewer reads the format directly, with no Publisher installation required. For choosing replacement software now that Publisher is retiring, see the Microsoft Publisher alternatives guide.

🔄 PUB Conversion Targets

Because PUB stores absolutely positioned objects, how well it converts depends entirely on whether the destination format shares that model. Fixed-layout targets preserve the page; flowing-text targets do not. This reference shows what survives:

Step-by-Step Conversion Guides

This page is a format reference. For instructions on each conversion, see the dedicated guides:

For batch processing across an archive, the Univik PUB Converter reads the format directly on Windows.

📜 Format Evolution 1991–2026

The .pub extension stayed constant for 35 years while the format underneath it changed repeatedly. This is why file age matters more than file size when troubleshooting:

❓ Frequently Asked Questions

📝 Summary