A PDF is a container, not one kind of content
A PDF can hold text, subset fonts, vector paths, photographs, scanned page images, forms, annotations, attachments, metadata, and incremental revisions. Two documents with the same page count may therefore have radically different sizes. “Compress PDF” means optimizing some combination of those internal objects.
Common compression methods
- Image recompression: decode embedded raster images, reduce their resolution or quality, and encode them again.
- Stream compression: apply efficient lossless compression to compatible content streams.
- Font subsetting: include only glyphs used by the document.
- Object cleanup: remove unused objects, duplicate resources, metadata, thumbnails, or old incremental revisions.
- Structural rewriting: organize objects and cross-reference data more compactly.
These techniques have different risks. Recompressing scans can blur small text; removing objects can affect workflows that depend on them; rewriting a signed PDF generally invalidates its digital signature.
Why results vary
A scanned report may be mostly high-resolution page images and offer substantial room for reduction. A text-first PDF with subset fonts and compressed streams may already be efficient. Re-encoding an already compressed image can save little and can even produce a larger stream. There is no honest fixed percentage that applies to every PDF.
What YesFreeTool’s compressor does
The PDF Compressor works locally with pdf-lib. It searches for raw image streams, attempts to decode them, limits successfully decoded images to an 800-pixel maximum dimension, and embeds them as low-quality JPEG data. Streams it cannot decode are left alone. The interface does not expose compression levels.
This method is most relevant to PDFs whose size is dominated by compatible raster images. It is not a general optimizer for every font, vector object, attachment, or revision, and it does not promise lossless output.
What to verify after compression
- Compare output and original sizes; a completed operation does not guarantee a smaller file.
- Zoom into small scanned text, diagrams, and signatures.
- Test links, forms, annotations, and printing where they matter.
- Do not rely on an altered copy retaining a valid digital signature.
- Keep the source until the new PDF has passed its intended workflow.
Related PDF workflows
If only certain pages are needed, splitting the PDF may reduce what you send without degrading retained pages. Combine selected documents with Merge PDF. Read when merging or splitting is the better operation.