How PDF Compression Works

Why is a 50-page text PDF only 200KB, while a 5-page scanned contract is 10MB? It all comes down to how PDFs store and compress data streams.

Text and Vector Objects vs. Raster Images

A native PDF (exported from Word or a web browser) stores text as mathematical instructions: "Go to coordinates X,Y and draw the letter 'A' using Helvetica". These instructions are incredibly lightweight.

A scanned PDF, however, is just a wrapper around a massive grid of pixels (a raster image). The PDF engine doesn't know there is text; it just sees a giant photograph of a piece of paper. You can estimate this with our PDF Size Estimator.

Stream Filters (FlateDecode)

To reduce file size, PDF utilizes "Filters" to encode streams. The most common is `/FlateDecode`, which is essentially standard zlib/deflate compression (the same algorithm used in `.zip` files).

The "Save As" Bloat

Because PDFs support incremental updates (appending changes to the end of the file rather than rewriting the whole thing), signing a document multiple times can lead to a bloated file, as old versions of objects remain in the file structure.