What is the BK file format?
.bk is a project file associated with Adobe FrameMaker. .bk files are book documents that reference a collection of component .fm chapter files rather than storing their content directly. Adobe FrameMaker is professional software for designing and publishing large and complex documents, including technical manuals, structured DITA publications, and multi-chapter books.
Adobe FrameMaker debuted in 1986 on UNIX systems, originally developed by Frame Technology Corporation. FrameMaker was later acquired by Adobe in 1995, which continues to maintain and update the product under a subscription model. FrameMaker uses its own document referencing system and conditional rendering engine that allows users to generate different versions of a document from the same input .bk file.
The .bk extension is the legacy DOS 8.3 short form of the modern .book extension; current FrameMaker versions default to .book. Note that .bk is also widely reused as a generic backup extension by many other applications, so confirm the file originated from a FrameMaker project before opening it.
Security & safety
RISK: LOWA FrameMaker .bk book file is a document/collection file, not an executable, so it poses little direct malware risk. The bigger issues are practical: it depends on its component chapter files (move them together) and needs paid FrameMaker to open, so data can become hard to access. If your .bk is actually a generic backup copy, it carries only the same risk as the original file it duplicates; if it's an e-book, treat it like any downloaded content. As always, scan files from untrusted sources.
Format details
in a nutshell- Generic backup file - Many programs save backup copies as .bk (like .bak) - a copy of another file, opened by renaming back to the original extension.
- Mobipocket eBook (.prc/.mobi family) - Some Mobipocket/early e-reader content used .bk for book content; open in Mobipocket Reader or convert with Calibre.
Programs that open BK files
Windows3 apps
Adobe FrameMaker
Paid File → Open the .bk/.book to open the whole publication; use File → Save as PDF (or Publish) to export. The only program that truly opens a FrameMaker book. 
Original program (if it's a backup .bk)
Built-in If the .bk is a generic backup, copy it and rename the extension back to the original (e.g. .docx/.dwg), then open in that program.
Calibre (if it's a Mobipocket e-book .bk)
Open-source Add the .bk to Calibre and convert/read it if it's actually e-book content. 
macOS1 app
Adobe FrameMaker
Paid FrameMaker is Windows-only today; on macOS you typically need a Windows VM, or ask the sender for a PDF/MIF export of the book. Technical details
deep spec| File role | Book/collection file that references component documents (chapters, generated TOC, and index) rather than embedding their content |
| Encoding | Binary FrameMaker format or MIF (Maker Interchange Format) plain text (ASCII/UTF-8), depending on how the book was saved |
| File signature | MIF-format books begin with `<MakerFile` or `<Book` at offset 0; binary books carry a version-dependent FrameMaker binary header |
| Typical file size | A few kilobytes - the file holds references and settings, not the text or graphics of component chapters |
| Compression | Uncompressed in both variants; binary books use a raw FrameMaker binary structure with no compression layer |
| Encryption | No built-in encryption or password protection; MIF-format books are readable as plain text in any editor |
| Component references | Lists file paths to external `.fm` chapter files plus generated files such as the table of contents and index |
| Book-wide settings | Stores pagination flow, chapter and figure numbering, cross-reference formats, and print/PDF output configuration |
| Conditional rendering | Integrates with FrameMaker's conditional text engine to produce multiple document variants (e.g. print vs. online) from a single book file |
| Extension origin | DOS 8.3 truncation of the full `.book` extension; FrameMaker 7 and later default to `.book` on all platforms |
| MIME type | `application/vnd.framemaker`; also mapped to generic `application/octet-stream` |
| Platform history | Originally UNIX/Sun workstations (1986 debut); subsequently available on Windows and macOS; current active support is Windows only |
| Portability | Not self-contained - moving the file without its companion `.fm` chapter files breaks all internal references and generated-file paths |
| Byte order | Little-endian for the binary FrameMaker variant; byte order is irrelevant for the MIF plain-text variant |
| Released | FrameMaker book feature (late 1980s/1990s); .bk was the DOS/8.3 short form of .book |
| Latest version | Tracks Adobe FrameMaker (2020 release / 2022 / subscription); modern books use .book |
| Specification | helpx.adobe.com |
BK conversions
Community Q&A
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