The HND project file format is based on the SQLite database format which includes various optimizations to speed-up disk reading and writing operations:

• When a large amount of data is deleted from the project (such as library items), it leaves behind empty space. This means the project file might be larger than needed;
• Frequent modifications (such as inserts, updates, and deletes) can cause the project file to become fragmented. This means that project operations can be slower than usual and the project file might be larger than needed;

Using the "Vacuum Project" command from HelpNDoc's "Tools" ribbon tab will optimize the currently opened HND project file by rebuilding it and repack its content into a minimal amount of disk space. This leads to smaller and faster HND project files.

Vacuuming HelpNDoc projects should be done from time to time.