PUB File Documentation

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Technical details

Feature	Value
Full name	Microsoft Publisher Document
File extension	.pub
MIME type	application/x-mspublisher
Format type	Print-layout document (proprietary binary)
Container / base format	OLE2 Compound File Binary (CFB)
Developer	Microsoft
Introduced	1991 (Publisher 1.0); OLE-based .pub since late 1990s
Specification	[MS-PUB] (partial Microsoft notes)
Open standard	No
Magic number	D0 CF 11 E0 A1 B1 1A E1
Byte order	Little-endian (CFB header fields)
Header size	512 bytes (fixed CFB header)
Layout model	Fixed print layout (not reflowable text)
Text engine	Quill; drawing via Escher/Office art layer
Native application	Microsoft Publisher (Windows only)
Support status	Retired by Microsoft after October 2026
Free import	LibreOffice Draw (partial fidelity)
Related extensions	.pdf, .docx, .indd, .cdr
Specification URL	[MS-PUB]

What is a PUB file?

A .pub file is a document made by Microsoft Publisher, Microsoft's entry-level desktop-publishing application for flyers, newsletters, brochures, postcards and greeting cards. Publisher first shipped in 1991 and has been bundled with higher-tier Office and Microsoft 365 editions, but it was never part of the core Word, Excel and PowerPoint trio, so many people receive a PUB without owning software that opens it.

Technically a PUB file is an OLE2 Compound File Binary: a proprietary binary container, described only partially in Microsoft's [MS-PUB] notes. It stores a fixed print layout — page geometry, positioned text frames, images and styling — rather than the reflowable text of a word processor, which is why it behaves like a layout file. Its MIME type is application/x-mspublisher. One fact dominates in 2026: Microsoft has announced that Publisher will no longer be supported after October 2026, which makes understanding and converting these files a live concern.

The Compound File Binary container

Every PUB file begins with the same 8-byte signature, D0 CF 11 E0 A1 B1 1A E1, the marker of Microsoft's Compound File Binary Format (CFB), also called OLE2. CFB is essentially a small file system inside one file. A fixed 512-byte header at offset 0 identifies the format and records the sector size and the starting locations of the internal allocation tables and the directory.

offset 0    D0 CF 11 E0 A1 B1 1A E1   CFB signature (8 bytes)
offset 8    [16-byte CLSID, usually zero]
offset 24   minor / major version
offset 28   FE FF                     byte-order mark (little-endian)
offset 30   sector shift (09 00 -> 512-byte sectors)
offset 44   number of FAT sectors
offset 48   first directory sector
offset 76   DIFAT (first 109 FAT sector locations)

The body is divided into fixed-size sectors (512 bytes in the common version 3 layout). A File Allocation Table, a mini-FAT for small streams, and a directory chain thread those sectors into named streams and storages. In OLE2 terms a stream is like a file and a storage is like a folder, so one CFB file can hold many independent byte streams — the same mechanism that stored a legacy DOC or XLS. Because Publisher shares this container with those formats, the magic bytes alone cannot tell you a file is Publisher; the identity comes from which streams the directory contains.

The Publisher-specific streams

What makes a CFB file a Publisher document is its set of named streams. Publisher separates the text engine, the drawing layer and document metadata into distinct storages within the compound file.

Stream / storage	What it holds
Quill text streams	Story text and its formatting, produced by Publisher's "Quill" text engine
Escher drawing stream	Shapes, lines and graphic objects (the Office art / Escher layer, shared with other Office apps)
Embedded images	Raster and vector graphics placed on the pages
Master pages / styles	Reusable page templates and text styles
\x05SummaryInformation	Standard OLE property stream: title, author, dates

The \x05SummaryInformation stream is the same document-property stream every OLE2 Office file carries, which is why Windows can show a PUB's title and author without Publisher installed. The layout and text, by contrast, live in Publisher's own Quill and Escher streams, and it is these that are poorly documented and hard for third-party software to interpret fully.

A fixed print layout, not reflowable text

Publisher is a page-layout tool, and PUB stores geometry accordingly. Text does not flow down an infinite document as it does in DOCX; it lives in text frames placed at exact coordinates on a page of fixed size, and stories can be threaded from one frame to the next. Images and shapes are likewise pinned to positions on the page. This is the same design goal as a PDF or an InDesign INDD file: preserve where every element sits so a printed sheet always looks identical. It is also why converting PUB to a reflowable format like DOCX rarely round-trips cleanly — a fixed layout has to be re-interpreted as flowing paragraphs, and positioning is lost.

Why opening a PUB without Publisher is hard

The closed binary format and the undocumented Quill and Escher streams mean third-party support is limited. LibreOffice Draw includes a PUB import filter, the most useful free option, but it renders complex layouts imperfectly because it has to reverse-engineer those streams. Adobe InDesign cannot open PUB at all; commercial migration plug-ins exist but are not free. In practice the reliable path is to open the file in Publisher itself and export to PDF, which captures the exact layout, and to accept partial fidelity from any other route.

The October 2026 retirement and what it means for your files

Microsoft has stated that Publisher will not be supported after October 2026. Microsoft 365 subscribers lose access to the app after that date; perpetual Office 2021 installations keep running but become unsupported. Microsoft's own guidance is to convert existing PUB files to PDF or Word before then. The practical risk is not that your files vanish but that the only software able to read them fully goes away, leaving documents locked in a format with weak third-party support. If you have PUB files that matter, open each in Publisher while it still runs and Save As PDF to preserve the exact layout, plus DOCX if you need the editable text.

Macro risk in an OLE2 document

Because PUB is an OLE2 Compound File Binary, it can carry the same thing other legacy binary Office documents can: a VBA macro project stored as its own stream inside the compound file. That is a genuine malware vector — a booby-trapped document can run code the moment macros are enabled — so treat an unexpected .pub the way you would an old .doc: do not enable macros unless you trust the sender. The other risk now is quieter but more likely to bite: with Publisher retiring, files left only in .pub may become hard to open, so converting the important ones to PDF is itself a data-safety measure. If you use an online PUB-to-PDF converter, avoid uploading confidential documents, since the file leaves your machine.

Frequently asked questions

Is Microsoft Publisher really being discontinued?

Yes. Microsoft has announced that Publisher will no longer be supported after October 2026. Microsoft 365 subscribers lose the app, perpetual Office 2021 copies keep working but unsupported, and Microsoft advises converting PUB files to PDF or Word beforehand.

Why does a PUB file share its signature with DOC and XLS?

All three use the same OLE2 Compound File Binary container, so all three start with D0 CF 11 E0 A1 B1 1A E1. The signature identifies the container, not the application. A tool tells them apart by inspecting the named streams inside — Publisher's Quill and Escher streams mark it as a PUB.

References

• Microsoft Support — Publisher will no longer be supported after October 2026
• Microsoft — [MS-PUB] Publisher binary file format notes
• The Document Foundation — LibreOffice Draw (PUB import)

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