Why Did My PDF Get Bigger After Compression?

The word compression describes an operation, not a guaranteed outcome. A compressor may decode an image and save it with a different encoder whose headers, color representation, or quality settings are larger than the original stream.

Saving the full PDF also writes a fresh cross-reference structure and document catalog. Those bytes are small in an image-heavy file but can outweigh tiny savings in a short text document.

Do not keep a larger output simply because it has “compressed” in its filename. Compare byte size and visual behavior, then retain the original when there is no benefit.

The most common cause is re-encoding an image that was already efficient. JPEG has a quality ceiling: re-saving an existing JPEG at a high quality setting frequently produces a larger stream than the source, because the encoder spends bits describing artifacts the previous encoder introduced.

Operations that add rather than remove also inflate a file. Running OCR adds a text layer plus the fonts needed to render it. Some tools embed page thumbnails, preserve revision history, or attach a fresh metadata block on every save.

Saving from a different application can embed full font sets where the original embedded only subsets — a change that is invisible on screen and expensive in bytes.

This is why a well-behaved compressor compares output against input and returns the original when it cannot do better, rather than handing back a larger file that merely has a different name.

Aggressive settings make this more likely, not less. A very low quality setting re-encodes harder, but it is still starting from decoded pixels rather than from the original source image. If the source was already encoded at a similar quality, the new stream can come out larger because the encoder has to describe artifacts that were not there the first time.