What is inside a PDF?
A PDF is not a flat image. It is a structured container that can hold text, fonts, vector drawings, images, and metadata, each stored with its own method. When someone says a PDF is ‘uncompressed,’ they usually mean the images inside were saved at high fidelity, the fonts were embedded in full, and no effort was made to trim metadata.
- Images: often the largest part: photos, scans, logos, charts.
- Fonts: the full typeface may be embedded even if you used only a few letters.
- Metadata: creation history, thumbnails, and editing traces that viewers never show.
- Structure: page tree and cross-references that can be streamlined.
The three techniques that actually shrink files
Almost all size reduction comes from these:
- 1. Image re-encoding: each image is re-saved with smarter JPEG settings (lower quality number, optimized resolution). A 300 DPI photo re-encoded at 150 DPI and 60–75% quality can be 60–80% smaller while still looking crisp on screen.
- 2. Font subsetting: instead of embedding an entire font (say, 200 KB for all glyphs), only the characters actually used are kept (often 10–20 KB). Readers never notice.
- 3. Metadata and object cleanup: redundant objects, old edit history, and unused resources are removed. This is safe and never affects appearance.
How Shrinklyy does it in your browser
Older tools send your PDF to a server. Shrinklyy does the same steps: rendering pages and rebuilding the file: but right on your device.
For fixed targets (e.g., 500 KB), the engine tries a sequence of quality settings from gentle to strong and keeps the highest quality that still fits. For Custom levels, it applies a preset (Low/Medium/High/Maximum) directly so you control the trade-off.
Because nothing is uploaded, there is no queue, no storage, and no file to delete later. The compressed bytes are created in memory and handed to you as a download.
Why some PDFs cannot shrink much further
A PDF that was already optimized: e.g., exported with ‘Smallest file size’ from Word or Acrobat: may have little slack left. Trying to force it from 400 KB to 100 KB will cost visible quality for minimal gain.
Conversely, a PDF that was built by inserting high-resolution JPGs at full size has enormous slack. Those files routinely shrink a lot with no visible loss.
Frequently asked questions
Quick answers to the questions readers ask most about this topic.
The output is a standard PDF that opens in every viewer. Adobe Reader, browsers, phones. Shrinklyy rebuilds the file with pdf-lib, which produces widely compatible PDFs.
Yes. Only unused glyphs are removed. Characters that appear in your document are fully preserved, so the text looks identical.
Text is vector-based and fonts are preserved precisely; photos are raster images that trade some detail for size. That trade-off is controllable via the compression level you pick.
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