What is the HDX file format?
An .hdx file is a binary compressed index sidecar used by the classic Windows WinHelp system. Two distinct products generated .hdx files:
- AutoCAD (Autodesk): The classic AutoCAD help system paired a file always named
ACAD.HDXwithACAD.HLPto provide fast keyword and full-text search inside the AutoCAD help viewer. - Delphi (Borland/Embarcadero): The Delphi IDE used an
.hdxindex built by theHelpInst.exetool from.kwfkeyword files to power integrated context-sensitive help.
In both cases the .hdx holds compressed topic names, keyword entries, and search strings extracted from the associated .hlp WinHelp file. It is read at runtime by the WinHelp engine (winhlp32.exe) and is entirely invisible to end users. The binary format is proprietary and undocumented; no standalone viewer exists.
The format is entirely obsolete. Microsoft replaced WinHelp with HTML Help (.chm) in 1997. AutoCAD dropped ACAD.HDX and ACAD.HLP in AutoCAD 2005 and later releases. Embarcadero's modern Delphi editions use a completely different help engine. WinHlp32.exe itself was removed from Windows Vista and later; on Windows 10 and 11 it is unavailable even as an optional component. Any .hdx file encountered today is purely an artifact of legacy software archives and serves no current purpose.
Security & safety
RISK: LOWA static binary index file that is not executable. No known malware vector. However, WinHelp (.HLP) files - the documents this indexes - can execute macros, which was historically exploited. HDX itself carries no code.
Format details
in a nutshell- HDR/HDX Hex dump index (debug tools) - Some older debuggers and disassemblers used .hdx for hexadecimal dump index files; unrelated to help systems.
Programs that open HDX files
Windows2 apps
AutoCAD (legacy, pre-2005)
Paid The file was used internally by AutoCAD's WinHelp-based help system; not opened manually. Modern AutoCAD does not use HDX.
WinHelp (winhlp32.exe, Windows XP era)
Built-in WinHelp reads HDX alongside HLP automatically; winhlp32.exe is not available on Windows 10/11 - the HDX is inaccessible on modern Windows without a VM. Technical details
deep spec| Format type | Binary compressed help keyword and search index |
| Encoding | Binary (compressed; internal structure undocumented) |
| Byte order | Little-endian (Windows-era format) |
| Typical file size | 100 KB - several MB (AutoCAD ACAD.HDX was several hundred KB) |
| AutoCAD variant | Always named ACAD.HDX; paired with ACAD.HLP in the AutoCAD installation directory |
| Delphi variant | Generated by HelpInst.exe from .kwf keyword source files for the Delphi IDE integrated help |
| Runtime role | Provides fast keyword lookup and full-text search for the associated .hlp WinHelp file; accessed only by winhlp32.exe |
| User accessibility | Invisible to end users; no standalone viewer exists for either variant |
| Target platform | Windows 3.1 through Windows XP (classic WinHelp era) |
| Superseded by | Microsoft HTML Help (.chm, 1997); AutoCAD dropped ACAD.HDX/ACAD.HLP in AutoCAD 2005 |
| Current runtime status | WinHlp32.exe removed from Windows Vista+; unavailable on Windows 10/11 without third-party workarounds - format is effectively inaccessible on modern systems |
| Released | AutoCAD variant: late 1980s-1990s; Delphi variant: Delphi 1 era, 1995 |
| Latest version | Obsolete - not used by any current AutoCAD or Delphi release |
HDX conversions
Community Q&A
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