PDF compression does not reduce quality in a single step. It applies a series of techniques, each of which trades some amount of quality for some amount of size reduction. Understanding what each technique removes allows you to choose the right compression level for each document rather than applying the maximum setting and hoping for the best.
What is actually in a large PDF
PDF file size is dominated by images. A PDF with only text is typically small — a hundred-page report with no graphics might be 500 KB. The same report with one high-resolution photograph per page might be 50 MB. The photographs are stored at their original resolution inside the PDF, even if they will never be printed larger than postcard size. The simplest compression technique is reducing that image resolution to something appropriate for the intended use.
Text in PDFs is stored as font data plus character references. If a PDF embeds the full font file for a font that only uses twelve characters, the unused characters represent wasted space. Font subsetting — removing unused characters from the embedded font — can reduce size without affecting appearance at all. Most good compressors perform font subsetting as a standard step.
PDF files accumulate structure overhead over time. When a document is edited and resaved repeatedly, the PDF format appends changes rather than rewriting the whole file. This produces revisions that contain the current version plus every previous edit. Linearizing or rebuilding the PDF removes this accumulated overhead and can reduce file size significantly on heavily-revised documents.
The three compression levels and what they remove
Light compression applies font subsetting, removes embedded metadata that is not needed for display, and optionally linearizes the file. Images are not downsampled. The quality of visual content is unchanged. This level typically reduces file size by 10 to 30 percent and is appropriate for any document where the PDF will be printed, edited further, or must maintain exact visual fidelity.
Standard compression adds image downsampling to an appropriate screen-viewing resolution — typically 150 DPI. This is sufficient for any document that will be read on screen, sent by email, or uploaded to a web system. Print-quality images at 300 DPI or higher are downsampled to screen quality. For most professional documents, standard compression produces a file that looks identical on screen while being 40 to 70 percent smaller.
Aggressive compression reduces images further to low-resolution display quality and applies lossy compression to any color images. The resulting PDF looks acceptable on small screens but shows visible degradation when zoomed or printed. This level is appropriate for archiving documents you need to access but rarely read in detail, or for documents where the only requirement is that text remains readable.
Documents that do not compress well
PDFs that are already compressed compress poorly a second time. Lossless compression algorithms reach diminishing returns quickly — the second pass on an already-compressed file often achieves less than five percent additional reduction. If a PDF is already at a reasonable size, additional compression may not be worth the quality tradeoff.
Scanned document PDFs compress differently than PDFs created from digital sources. A scanned page is a photograph of paper. At any given resolution, the file size is determined by the scan resolution and the amount of visual noise in the image. Scans of clean white paper with black text compress well with lossless methods. Scans of aged or slightly yellow paper contain color variation that compresses less efficiently.
Preserving quality where it matters
Before compressing, identify which elements in the document must be preserved at full quality. Photographs in a portfolio require high fidelity. Charts and graphs in a report require legible labels. Signatures and official seals must remain clear for verification. If these elements exist in the document, use light compression or set a minimum image resolution that preserves their legibility.
If only part of the document contains quality-critical content, consider splitting the PDF into sections, compressing the text-heavy sections aggressively, and keeping the image-heavy sections at light compression, then merging the results. This approach is more work than a single-pass compression but allows different quality settings for different content types within one document.
Compressing in Filum
Filum's PDF compression uses Gotenberg with LibreOffice, which applies font subsetting, metadata removal, and image downsampling in a single pass. The compression settings let you choose between three levels. The quality score after compression reflects the measured output quality against the input, so you can see objectively how much visual information was removed rather than guessing.
For email-size requirements, standard compression typically reduces a 10 MB document to under 3 MB. For storage archiving, aggressive compression typically reduces the same document to under 1 MB. The file is processed server-side and deleted 60 minutes after the download is confirmed.