Why PDFs are sometimes unnecessarily large
A 50-page report with few graphics should not be 80 MB, yet it happens regularly. The most common causes: images embedded at too high a resolution, unneeded metadata, fonts embedded in multiple variants, or uncompressed scans. Before reducing a PDF's size, it helps to understand what is driving the file size.
The main factors behind PDF file size
Images and photos
Images are the biggest factor in most cases. A high-resolution photo at 4000x3000 pixels takes up several megabytes even as an embedded JPEG. For screen display, a resolution of 72-150 dpi is sufficient; for print, 150-300 dpi. Anything beyond that wastes space in most PDFs.
Scanned pages
Scans are often the biggest problem. A scanned A4 page at 600 dpi as an uncompressed image can easily take up 10-20 MB. After compression, often only 200-500 KB remain, with no perceptible quality loss for text documents.
Embedded fonts
Fonts are embedded in PDFs so the document looks the same on every device. When many font variants are fully embedded (Regular, Bold, Italic, Bold Italic), this adds up quickly.
Unneeded metadata and resources
PDFs can contain hidden information: edit history, thumbnails, comments, embedded files. These contribute to file size while being invisible to the average reader.
How PDF compression works
With compress PDF, several optimization steps are performed automatically:
- Images are downsampled to a defined maximum resolution
- Images are re-encoded using optimized compression algorithms
- Unneeded resources and metadata are removed
- Fonts are embedded with only the characters actually used (subsetting)
The result is typically significantly smaller, 40-70% for text-heavy documents, sometimes over 80% for image-heavy documents.
When do you see quality loss?
With text, there is no visible quality loss from compression, text is vector-based and retains full quality. With images, it depends on the compression strength chosen:
- Moderate compression: barely perceptible difference during normal viewing
- Strong compression: JPEG artifacts visible, especially in large areas with gradual gradients
- Extreme compression: clearly visible quality loss, only acceptable when image sharpness is not important
For documents that will be printed, choose more moderate compression. For purely digital documents, stronger compression often works perfectly.
Optimize, not just compress
Beyond pure compression, there are additional measures that reduce file size:
- Remove unnecessary pages before compressing, with delete PDF pages
- Merge multiple small individual documents and then compress, instead of many separate files
- For scans: run OCR first, then compress, the text layer is small and reduces the need for very high image resolution
Compare before and after
After compression, it is worth quickly reviewing the result. Do all pages still look good? Are diagrams and graphics still sharp? Is text still easily readable? If you want to check changes on specific pages, the original and compressed version can be compared side by side using compare PDF.
Typical results after compression
- Word-exported PDF with some logos: 8 MB → 2-3 MB
- Scan of a 10-page form: 20 MB → 1-2 MB
- Product catalog with photos: 150 MB → 20-40 MB
- Simple text document: 500 KB → 150-200 KB
Conclusion
PDFs can be significantly reduced in size in most cases without the content suffering. The key is choosing the right compression settings and understanding which elements are driving the file size. For everyday documents, a good online compression tool delivers fast and reliable results.