What makes a PDF large?
PDF file size is mostly driven by embedded images. A scanned document is essentially a collection of images stitched together, and those images can be large if they were scanned at high resolution. Similarly, a PDF exported from a design application might contain full-resolution photos embedded at print quality.
Text itself compresses very efficiently. A 50-page PDF with nothing but text might be only 200KB. The same document with a few full-page stock photos could be 20MB.
Fonts, form fields, annotations, and metadata also contribute to file size, but usually much less than images.
How PDF compression works
PDF compressors primarily target the embedded images. They reduce the resolution (DPI) of images from print quality (300 DPI) down to screen quality (72–150 DPI) and apply additional JPEG or WebP compression. For screen reading, images at 96–120 DPI are practically indistinguishable from 300 DPI ones.
For text-only PDFs, compression tools can remove redundant data from the file structure, optimize the cross-reference table, and remove unused objects — but the gains are typically modest (10–30%).
The Compress PDF tool on Irreva uses pdf-lib to rebuild the file with optimized image data. Processing runs in your browser.
Choosing the right compression level
For documents you'll share digitally and read on screen — reports, contracts, portfolios — medium compression (targeting 96–150 DPI for images) gives you substantial size reduction with no visible quality loss at normal reading zoom.
For documents that will be printed, keep image DPI at 150 or higher to preserve sharpness on paper. Compressing to 72 DPI will look fine on screen but can appear blurry when printed at full size.
For documents that are mostly text with a few images, the file is already close to minimum size. Don't expect dramatic results from compression — try splitting the document first if you only need to share certain pages.
- Screen / email (72–96 DPI): maximum compression, smallest file
- Web / shared documents (120–150 DPI): good balance for digital use
- Print quality (200–300 DPI): minimal compression, large file
When compression can't help much
If your PDF is already compressed, running it through a compressor again will produce minimal gains and might slightly reduce quality in image areas without meaningful file size reduction.
Text-only PDFs are already near-optimal. If a text PDF seems large, check whether it was scanned (pages are actually images) rather than digitally created.
PDFs with many embedded fonts can sometimes be reduced by subsetting the fonts — only embedding the characters actually used — but not all compressors support this.