| 1. | .hda | Document file (HotDocs) |
| 2. | .hdf | Bitmap graphics Hierarchical Data Format (NCSA) ◊ Help file (Help Development Kit) |
| 3. | .hdl | Alternate download file listing (Procomm Plus) |
| 4. | .hdml | Handheld Device Markup Language |
| 5. | .hdr | Database header (Pc-File+) ◊ Datafile (Egret) ◊ Header information ◊ Message header (Procomm Plus - 1st Reader) |
| 6. | .hds | Hierarchical Data System |
| 7. | .hdw | Vector graphics (Harvard Draw) |
| 8. | .hdx | Help index (AutoCAD - Zortech C++) |
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