Scanned PDF Is Too Large — How to Reduce Scanned PDF Size
A PDF created by scanning physical documents is extremely large — often 5–50 MB per page — and needs to be reduced for emailing or uploading.
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Why Scanned PDFs Are So Large
A scanned PDF is essentially a series of high-resolution photographs — one per page. At 300 DPI, a standard A4 page produces a TIFF image of approximately 25 MB. PDFs can embed these at various compression levels, but even with good compression, a 10-page scanned document can easily be 30–50 MB.
The main factors determining scanned PDF size:
| Factor | Impact |
|---|---|
| Scanner DPI setting | 600 DPI produces 4× more data than 300 DPI |
| Colour vs greyscale | Colour is 3× larger than greyscale |
| Internal compression | "None" creates enormous files; JPEG compression reduces dramatically |
| Page count | Each page multiplies the total |
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Fix 1: Compress the Existing Scanned PDF (Fastest)
If you already have the scanned PDF:
- Open Compress PDF
- Upload your scanned PDF
- Select Medium compression
- Download
What happens: The tool resamples the embedded images from their original high DPI to 150 DPI (screen-appropriate). For a document you will read on a screen or submit to a portal, 150 DPI is indistinguishable from 300 DPI.
Expected result: A 50 MB scanned PDF typically reduces to 5–10 MB with Medium compression — a 80–90% reduction.
High compression reduces further (to 96 DPI) — text may appear slightly soft when zoomed in, but is perfectly readable.
Fix 2: Scan at Lower Settings Next Time
If you are doing the scanning yourself and the documents are not yet scanned:
Optimal scanner settings for documents destined for email/portals:
- Resolution: 200–300 DPI (not 600 DPI)
- Colour mode: Greyscale for text documents (not Colour, unless colour is necessary)
- Format: PDF (not TIFF)
- Compression: JPEG inside PDF or compressed TIFF
A text document scanned at 200 DPI in greyscale is typically 100–300 KB per page — compared to 3–8 MB per page at 600 DPI in colour.
Fix 3: OCR + Re-Compress
If the PDF will be archived or searched, apply OCR first:
- Apply OCR (Google Drive → Open in Docs, or Adobe Acrobat)
- Then compress the OCR'd PDF
OCR adds a tiny invisible text layer. The compression applies to the image layer. The result is small, searchable, and readable.
Frequently asked questions
What is the best scan resolution for document PDFs?
200–300 DPI in greyscale for text-only documents. 300 DPI in greyscale for documents with small text or fine details. 300 DPI in colour only if the document has important colour information (charts, photos). 600 DPI is almost never necessary for documents and creates huge files.
Can I reduce scanned PDF size without losing text readability?
Yes. Compressing from 300 DPI to 150 DPI (Medium compression in the Compress PDF tool) produces no visible change in text readability on screen. Text remains sharp and legible. High compression (to 96 DPI) is still readable for most documents but text may appear slightly soft at high zoom levels.
How do I make a scanned PDF searchable?
Apply OCR (Optical Character Recognition) to the scanned PDF. Free options: upload to Google Drive and open with Google Docs (automatic OCR), then download as PDF. Paid option: Adobe Acrobat's Recognize Text feature. After OCR, the PDF has a searchable text layer over the scanned image.