What is the HLP file format?
.hlp is the Windows Help File format (WinHelp) - a binary compiled help document containing information about programs and their usage. Help files serve as documentation for users on how to use Windows and system applications. .hlp files are compiled from RTF source topics and an .hpj project file using Microsoft Help Workshop (HCW.EXE) and viewed with the WINHLP32.EXE engine. Files compiled for very different Windows versions may not be compatible with one another.
The WinHelp system is built from RTF-derived source files that offer basic text formatting and font colors, and support images and hyperlinks. RTF source does not support animations or interactive elements. The .hlp format made its debut with Windows 3.0 in 1990, and WinHelp 4.0 - shipped with Windows 95 in 1995 - added secondary windows, browse sequences, and full-text search.
The compiled .hlp is a proprietary binary container (not a collection of RTF files) that holds compressed topic data, a context map, embedded bitmaps, and optional full-text indexes inside an internal B-tree directory. It begins with the 4-byte magic 3F 5F 03 00.
Since Windows Vista, the WinHelp engine is no longer installed by default. Microsoft offered a downloadable WinHlp32 add-on for Vista and Windows 7, but withdrew it for Windows 10 and 11 - meaning most .hlp files cannot be opened on current Windows without third-party viewers. The format was superseded by HTML Help (.chm) in 1997.
Security & safety
RISK: MEDIUMWinHelp .hlp files can contain macros (the WinHelp macro language) and were a documented attack surface - one reason Microsoft removed the engine from modern Windows. Do NOT open .hlp files from untrusted sources, especially with the old WinHlp32 engine, which has unpatched vulnerabilities. For trusted legacy files prefer converting them to CHM/HTML/PDF, or use a sandboxed third-party viewer. Also beware "free .hlp viewer/fixer" download sites, which are a common adware/PUP vector.
Format details
in a nutshellPrograms that open HLP files
Windows4 apps
Adobe RoboHelp
Paid Open the legacy WinHelp project to view and convert it; modern RoboHelp outputs HTML/CHM/responsive help. 
Help Explorer Viewer
Paid Third-party viewer that opens legacy .hlp (and .chm) on modern Windows without the removed WinHelp engine. 
WinHlp32 (legacy, Windows XP/Vista/7 only)
Built-in On XP it is built in; on Vista/7/8 the downloadable WinHlp32 add-on opened .hlp. Microsoft withdrew it for Windows 10/11 - no longer a supported path.
HelpSmith
Paid Import a WinHelp .hlp/.hpj project and re-output it as CHM, HTML or PDF to read on modern Windows. 
macOS1 app
Linux1 app
Help Explorer Viewer
Paid Cross-platform WinHelp/CHM viewer; opens .hlp where no native Windows engine exists. Technical details
deep spec| File magic | 3F 5F 03 00 (little-endian 0x00035F3F) at offset 0 - the WinHelp 3.1/4.0 file header signature |
| Encoding | Binary, little-endian |
| Container structure | Proprietary WinHelp internal file system: a B-tree directory of named streams (|SYSTEM, |TOPIC, |Phrases, |CTXOMAP, |TTLBTREE, bitmap streams) |
| Compression | Optional LZ77/Hall compression on topic data blocks, controlled by the COMPRESS= option in the .hpj project file at compile time |
| Source files | Compiled from .rtf topic files and an .hpj project file (optionally with a .cnt contents file) using HCW.EXE or HCRTF.EXE (Help Workshop) |
| WinHelp engine | WINHLP32.EXE (32-bit) / WINHELP.EXE (16-bit); not included in Windows Vista or later by default |
| Typical file size | 20 KB - 5 MB; occasionally larger for comprehensive reference manuals |
| Version history | WinHelp 3.0 → WinHelp 3.1 (HC31 compiler) → WinHelp 4.0 (added secondary windows, browse sequences, full-text search, training cards) |
| Context-sensitive help | Integer context IDs are mapped to topic addresses via the |CTXOMAP stream, enabling F1-key integration from host applications |
| Embedded graphics | Supports Windows BMP bitmaps, SHG (Segmented Hypergraphics with hotspot images), and MRB (multi-resolution bitmaps) |
| Companion files | .cnt (contents tree), .gid (full-text index cache), .fts and .ftg (full-text search data) - generated alongside the .hlp file |
| Integrity | No CRC or checksum - file corruption causes the file to fail to open in WINHLP32.EXE without error detail |
| Superseded by | HTML Help (.chm, introduced 1997); later Microsoft Help 2 (.hxs) and Microsoft Help Viewer for online documentation |
| MIME type | application/winhlp |
| Released | 1990 (WinHelp, Windows 3.0); HLP 3.1 then WinHelp 4.0 with Windows 95 (1995) |
| Latest version | WinHelp 4.0 (1995); format frozen - deprecated after Windows XP, no engine in Vista+ by default |
HLP conversions
Community Q&A
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