A PDF that weighs 15 MB is annoying to email, slow to upload and often rejected by online forms. The good news: most PDFs can be made much smaller. The trick is knowing why your file is large, because that decides which kind of compression will work.
Why are some PDFs so big?
- Scanned pages and photos. A scanned page is a picture. At 300 DPI in colour, a single A4 page can weigh 1–3 MB.
- Embedded fonts. Some programs embed complete font files, sometimes several times.
- Inefficient structure. Files edited many times can contain unused objects and uncompressed data.
Two kinds of compression
1. Lossless (structural) compression
This rewrites the internal structure of the file — packing objects together and removing redundancy — without touching text or images. Quality is identical and text stays selectable. It works best on text documents exported from Word or similar programs, although the savings are often modest (0–30%).
2. Image-based compression
Each page is re-rendered as an optimized JPEG image. This is extremely effective for scans and photo-heavy files, often cutting the size by 50–90%. The trade-off: the text is no longer selectable, because the page is now a picture.
Step by step with PDFly
- Open the Compress PDF tool.
- Add your file. It is processed in your browser; nothing is uploaded.
- Choose Basic for text documents, Medium for scans you need to keep readable, or Strong when size matters most.
- Press Compress. PDFly shows the new size and the percentage saved. If the result would be larger than your original, your original is kept.
Tips for even smaller files
- Scan at 150–200 DPI instead of 600 DPI. For documents that will be read on screen or printed on an office printer, it is plenty.
- Use greyscale for black-and-white documents; colour triples the data.
- Compress photos before turning them into a PDF with Compress image.
- Remove unnecessary pages — blank backs of scanned sheets add weight for nothing.
When compression is not enough
If a portal limits files to 1 MB and your 40-page scan is still 3 MB after strong compression, consider sending only the pages that are requested, or splitting the document into parts. Our guide on reducing PDF size for email covers these strategies in detail.