How to Compress a Scanned PDF Without Losing Quality (2026)
Scanned PDFs are fundamentally different from regular PDFs — instead of text and vector graphics, every page is stored as a full image. A single A4 page scanned at 300 DPI in colour can be 1–3MB on its own, making a 10-page document 20–30MB before you've done anything. This guide covers why scanned PDFs are large, the best compression approach for each scenario, and how to scan smarter to avoid the problem entirely.
Why Scanned PDFs Are So Large
When you scan a physical document, the scanner converts it to a bitmap image — a grid of coloured pixels. Unlike a Word document or a digitally created PDF (which stores text as compact vector instructions), a scanned PDF stores the entire page as raw pixel data.
A standard A4 page at 300 DPI has 2,480 × 3,507 pixels = about 8.7 million pixels. In full colour (3 bytes per pixel), that's roughly 26MB of raw data per page before any compression. Even with JPEG compression, a single colour page at 300 DPI easily reaches 1–3MB.
This is why a 10-page scanned payslip can be 20MB while a 100-page text PDF created in Word might be under 2MB.
DPI and Colour Mode: The Two Biggest Factors
Two settings at scan time have more impact on file size than anything else:
DPI (dots per inch): This controls resolution. 300 DPI is the standard for printing — but for a document that will only ever be read on screen, 150 DPI is sufficient and produces files roughly 4× smaller (halving DPI in both dimensions reduces pixel count by 4×). 100–120 DPI is legible for standard text documents.
Colour mode: Colour (RGB) stores 3 bytes per pixel. Grayscale stores 1 byte. Black and white (1-bit) stores 1 bit. For documents with no colour content — contracts, payslips, transcripts, letters — scanning in black and white at 150 DPI typically produces files under 100KB per page, compared to 1–3MB for 300 DPI colour.
File Size by Scan Setting (per A4 page, approximate)
300 DPI colour: 1–3MB per page
300 DPI grayscale: 400KB–1MB per page
150 DPI colour: 300–700KB per page
150 DPI grayscale: 80–250KB per page
150 DPI black & white: 20–80KB per page
How to Compress Scanned PDFs Effectively
1
Upload to ShrinkPDF
Go to ShrinkPDF.fyi and upload your scanned PDF. No size limits — works on files of any size.
2
Select Maximum Compression
For scanned documents, Maximum compression reduces embedded image resolution from 300 DPI to approximately 150 DPI — still clearly readable on screen — while keeping all text content intact. This typically achieves 70–85% size reduction.
3
Download and Check Legibility
Open the compressed PDF and zoom in to check fine print, signatures, and small text. For IC numbers, account numbers, or medical prescriptions where fine detail matters, make sure the compressed version is still clearly legible before submitting.
4
Re-scan if Quality Is Not Acceptable
If Maximum compression makes the scanned text too blurry for your use case, re-scan the document at 150 DPI in black and white. This produces a smaller file from the start that compresses further with less quality loss.
Best Scan Settings to Avoid the Problem
The best time to control scanned PDF size is before you scan. These settings produce smaller files without sacrificing readability:
Recommended Scan Settings for Documents
Resolution: 150 DPI for text-only documents (payslips, contracts, letters). 200–300 DPI only if you need to read very fine print or preserve handwriting detail.
Colour mode: Black and White for any document without colour content. Grayscale if you want slightly better image quality than pure B&W. Colour only for photos, certificates with coloured seals, or documents where colour is meaningful.
Output format: Scan directly to PDF rather than TIFF or BMP and then convert — the intermediate step adds file size.
Pages: Scan only the pages you actually need to submit. Blank backs, cover pages, and staple marks don't need to be included.
Best Mobile Scanning Apps for Smaller Files
If you scan on your phone, the app you choose makes a significant difference:
Microsoft Lens (free): Produces well-compressed PDFs by default. The document mode automatically removes shadows and flattens the image, reducing file size compared to a raw photo. Recommended for most users.
Adobe Scan (free): Good quality, but default settings lean toward higher resolution. Select "Normal" quality in settings rather than "High" to keep file sizes manageable.
CamScanner (free tier): Standard quality mode produces reasonable file sizes. Avoid the "HD" mode for documents that just need to be legible.
iPhone built-in scanner (Files app / Notes): Convenient but tends to produce larger files than dedicated apps. Good for occasional quick scans; use a dedicated app for regular portal submissions.
What Size Reduction Can You Expect?
Typical Compression Results for Scanned PDFs
20MB colour scan (300 DPI, 10 pages): Compresses to approximately 3–6MB — 70–85% reduction
50MB colour scan (300 DPI, 25 pages): Compresses to approximately 8–15MB — 70–85% reduction
10MB grayscale scan (300 DPI): Compresses to approximately 2–4MB — 60–80% reduction
5MB black & white scan (300 DPI): Compresses to approximately 1–2MB — 60–75% reduction
Results vary by scanner, content density, and how much compression was already applied by the scanning app.
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Will compressing a scanned PDF make the text unreadable? ▼
Maximum compression reduces image resolution from 300 DPI to approximately 150 DPI. For standard text documents — payslips, contracts, letters — this is still clearly readable on screen. Zoom in to fine print after compressing to verify. If the result is not sharp enough, re-scan at 150 DPI instead of 300 DPI; starting at the right resolution produces better results.
What is the best DPI to scan a document for portal submission? ▼
150 DPI in black and white is the sweet spot for text documents submitted to government portals, universities, or job applications. It produces files typically under 100KB per page, well within the 1–2MB limits of most portals, with no compression needed. Only use 300 DPI if you need to preserve fine detail like small handwriting or medical prescriptions.
Can I compress a scanned PDF on my phone? ▼
Yes. ShrinkPDF works in mobile Chrome and Safari. For small to medium scanned documents (under 20MB), mobile compression is fast and reliable. For large scanned files (50MB+), a desktop browser is recommended for better speed and stability.
Why does ShrinkPDF not upload my scanned documents to a server? ▼
ShrinkPDF processes all files using JavaScript in your browser. Your scanned documents — which may contain IC copies, salary details, medical records — never leave your device. This is particularly important for scanned documents which often contain more sensitive personal information than digitally created PDFs.