Incremental Update

Definition

When most applications edit a PDF, they do not rewrite the whole file; they append the changed objects at the end and add a new index pointing at them. Each save therefore makes the file longer, and the objects replaced by the newer version are still physically present in the file. A document edited twenty times carries twenty update layers, and shrinking it means rebuilding the file once so only the current generation survives.

Example

A contract drafted over weeks, opened and saved by an editor several times a day. Its size has grown far past what its pages justify, and the earlier versions of the edited paragraphs are still inside the file — recoverable by anyone who looks. Rebuilding the document removes the superseded layers, and the file shrinks while every visible page stays identical.

In the PDFLean tools

• compress a PDF — the rebuild that removes the superseded layers
• decompress a PDF — the related structural rewrite, in the opposite direction

Related reading

• Object Streams — the container structure the rebuild repacks
• Cross-Reference Table — how each update layer is indexed
• Compress a PDF Without Uploading It — where the rebuild is explained step by step