Scans are photos, not text
Scanning captures every dot as an image. A Word export stores the letter “A” as one character, while a scan stores it as hundreds of pixels. Across a whole page, that’s why the weight explodes.
This is also why text search does not work in many scanned PDFs: there is no text layer to search, only an image. (OCR can add one, but it does not reduce size.)
The three dials that set size: DPI, colour, format
These are the settings that matter at the moment you scan or photograph:
- DPI (dots per inch): 300 DPI captures fine detail for photos, but for text documents 150–200 DPI is plenty and roughly halves the file. Below 150 DPI, small print starts to suffer.
- Colour mode: colour captures three channels; grayscale one; black-and-white one bit. A colour scan can be 3–6× larger than grayscale for the same page. Use grayscale for certificates and black-and-white for pure text.
- Format: scanning directly to PDF is fine, but saving as high-quality JPG first and then converting to PDF adds an extra compression step. Better to scan to PDF and then compress once with a proper tool.
Fixing an already-large scanned PDF
You do not need to re-scan if you already have the heavy file. Compression handles it well because images compress efficiently.
With Shrinklyy, image-heavy scans see the largest reductions: a 10 MB colour scan routinely becomes 500 KB–1 MB with readable results. If you need a strict portal size like 200 KB, grayscale scans have a far better chance of getting there.
- Try Custom → Medium first, preview, then step to High only if you need more savings.
- If the result must be under 200 KB and looks soft, re-scan in grayscale rather than forcing extreme compression.
- Avoid taking a photo of a printout of a scan: each generation adds loss.
How much difference do settings make?
Rough real-world comparison for a single A4 text page:
- 300 DPI colour: several megabytes per page
- 200 DPI grayscale: much smaller: often well under a megabyte
- 150 DPI black-and-white: often under 150 KB
- After Shrinklyy compression, those converge to roughly 80–500 KB total depending on target: but starting leaner always gives a cleaner result.
Frequently asked questions
Quick answers to the questions readers ask most about this topic.
Not if you stay at 150–200 DPI for text. Small fonts (8–9 pt) need 150 DPI minimum; normal 11–12 pt text is fine at 150. Test by zooming to 150%, if the smallest text is still crisp, you are good.
Phone cameras work if you use a scan app that crops, straightens, and saves as PDF. Avoid casual photos with shadows and skewed perspective: they compress less efficiently and look unprofessional.
OCR adds a searchable text layer but does not shrink the underlying image. Use compression to reduce size; add OCR separately if you need searchability.
Try these tools
Matched to the workflow described in this guide.