1. The images inside are already compressed JPEGs.
PDFs do not store images in raw uncompressed pixels—they encapsulate existing JPEG or PNG binary streams directly. If you paste pre-compressed 80% JPEG photos into a PDF, re-compressing them using standard zip/deflate algorithms produces virtually 0% file size reduction.
To shrink an image-heavy PDF, the compressor must physically downsample the image resolution (e.g. from 300 DPI to 150 DPI) or lower the JPEG DCT quantization matrix, which Zcompressor performs automatically.
2. Embedded full font subsets and vector artwork.
When you export a PDF from Microsoft Word, Adobe InDesign, or Canva, the software frequently embeds entire font families (TTF/OTF font files) inside the PDF file structure.
A single custom font family can add 2 MB to 5 MB of pure vector font data. Standard image compression algorithms cannot strip embedded fonts without breaking document rendering.
3. High DPI scanner color depths (24-bit RGB vs 8-bit Grayscale).
Scanning a single white paper sheet in 600 DPI full color creates an 8 MB bitmap file. Even aggressive compression can only shrink this down to ~1.2 MB—still far above the 200 KB or 300 KB limit required by government portals.
Scanning at 150–200 DPI in Grayscale reduces the raw input size by 80% before compression even begins.
4. Hidden PDF version history and structural bloat.
PDFs edited multiple times in desktop PDF editors often retain old version revisions, hidden annotations, and unused object streams inside the file trailer.
Re-processing the PDF through Zcompressor strips orphaned streams and rebuilds a clean cross-reference table.
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Frequently Asked Questions.
How can I force my PDF under 200 KB if it won't compress?
Re-scan the original paper document at 150 DPI Grayscale mode, or convert multi-page color scans to 8-bit monochrome before running them through Zcompressor.