Why scanned PDFs behave differently
A digitally created PDF stores text as text — tiny, infinitely sharp, and almost free to compress. A scanned PDF stores each page as a picture. Every compression decision is therefore an image-quality decision: push too hard and letters smear, stamps blur, and signatures turn to mush.
This is why a 3-page scan can be 6MB while a 300-page ebook is 2MB. It's also why "compress to 200KB" is easy for a contract typed in Word and genuinely hard for the same contract scanned in color at 600 DPI.
The readability sweet spot: DPI, color, quality
- DPI: 300 DPI is the common standard for document scanning; 150–200 DPI is usually still perfectly readable and roughly 2–4× smaller. Below ~120 DPI, small print degrades quickly.
- Color: Grayscale cuts size to roughly a third of color. Black-and-white (1-bit) is smaller still but can destroy pencil marks and light stamps — test first.
- JPEG quality: The invisible dial inside most compressors. Down to moderate quality, text stays readable; at very low quality you'll see blocky artifacts around letters.
A safe compression workflow
- Upload to the compressor and choose your portal's target.
- Start with the Gentle level. If the result hits your target — done, you kept maximum quality.
- If it misses, step down to Balanced, then Strong. The tool tells you honestly when a target can't be reached at readable quality.
- If even Strong misses, the target is too aggressive for that page count — split the document or remove non-essential pages, then compress the smaller piece.
Rescuing a scan that won't compress
Some scans arrive broken: skewed, low-contrast phone photos at odd angles. If compression keeps failing, fix the source instead. Re-scan flat on a scanner (or with a scanning app that de-skews), choose grayscale, 150–200 DPI. If you only have a paper copy and a phone, take photos in bright, even light, directly overhead — then convert with JPG to PDF, which also optimizes the images.
How to verify readability before submitting
Open the compressed PDF and zoom to 150%. Check the smallest text on the page, every signature, every stamp or seal, and any handwriting. If you have to squint, the reviewer will too — and for government or legal submissions, an unreadable document can mean rejection or a request to resubmit. When in doubt, choose the bigger, clearer file and split the document to meet the limit.