Scanned PDFTroubleshootingHow-To⏱ 8 min readUpdated 2026-04-12by Yee Wee

Why Is My Scanned PDF So Large? Causes and Fixes (2026)

You scanned a 3-page payslip and the PDF is 15MB. Your transcript scan is 25MB. A single IC copy is 4MB. Scanned PDFs are almost always larger than they need to be — sometimes by a factor of 10. This guide explains exactly why, and how to fix it without re-scanning.

Why Scanned PDFs Are So Large

When you scan a document, your scanner or phone camera captures every pixel of the page as a raster image — a grid of dots. Unlike a digitally-typed PDF where text is stored as mathematical instructions (tiny), a scanned PDF stores the entire page as a photograph. A full-colour photograph of an A4 page at high resolution contains millions of pixels, which adds up to megabytes of data.

To put a number on it: a standard A4 page at 300 DPI is roughly 2,480 × 3,507 pixels — about 8.7 million pixels. In full colour at 3 bytes per pixel, that's around 26MB of raw image data for a single page before any compression. This is why a 10-page scanned payslip can be 20MB while a 100-page text document created in Word might be under 2MB.

The three factors that determine scanned PDF size:

Resolution (DPI): More dots per inch = more pixels = larger file. Doubling DPI roughly quadruples file size.
Colour mode: Colour stores three values per pixel (red, green, blue). Grayscale stores one. Colour is ~3× larger than grayscale at the same DPI.
Compression applied at scan: Some scanner apps apply JPEG compression before saving; others save raw image data. Less compression at scan = larger file.

The Main Culprit: Scanner DPI Settings

Most scanners and scanner apps default to 300 DPI — the standard for print-quality output. For documents that will only be viewed on screen (government portals, email, job applications), 300 DPI is 2–4× higher resolution than necessary, and produces files 4–16× larger than a 150 DPI scan.

File size comparison by DPI — single A4 page, colour

72 DPI: ~50–150KB — screen minimum quality
96 DPI: ~100–300KB — acceptable screen quality
150 DPI: ~300KB–1MB — good quality, recommended for submissions
300 DPI: ~1–4MB — print quality, default for most scanners
600 DPI: ~4–16MB — high-resolution archiving

A 300 DPI scan that's 3MB per page becomes approximately 300KB per page at 150 DPI — a 90% reduction just from changing the DPI setting, with no visible quality difference when viewed on screen.

Why Colour Scanning Makes Files Much Larger

Colour images store three data channels per pixel: red, green, and blue (RGB). Grayscale images store one channel — brightness only. This means a colour scan is approximately 3× larger than an identical grayscale scan at the same DPI, before any compression is applied.

For most official documents — payslips, transcripts, letters, IC copies, bank statements — colour provides no meaningful information that grayscale wouldn't convey. The text is black, the background is white, and any logos or stamps are legible in grayscale. Switching to grayscale scanning reduces file size by 60–70% compared to colour at the same DPI.

When colour scanning is actually necessary

Certificates with coloured official seals where the colour is part of the verification
Identity documents where the photo must be in colour
Documents with colour-coded content (charts, tables with colour meaning)
Portfolio or design work where colour accuracy is the point

For everything else — most government submissions, job applications, financial documents — grayscale is equally accepted and dramatically smaller.

How Different Scanner Apps Handle File Size

Different scanner apps produce very different file sizes for the same document, even at equivalent quality settings. Here's what to expect:

App	Default file size (A4, colour)	Notes
Microsoft Lens	200–500KB	Best default compression, highly recommended
iPhone Notes scanner	300KB–1MB	Good quality with reasonable compression
CamScanner (Standard)	500KB–2MB	Varies by quality setting; HD mode is much larger
Adobe Scan	1–3MB	High quality default, limited compression control
Flatbed scanner default	2–8MB	300 DPI colour default — always change this
Phone camera (direct photo)	3–10MB	JPEG not optimised for PDF embedding

Fix Without Re-Scanning: Compression

If you can't re-scan the original document (it belongs to someone else, you no longer have access, or the scan was done externally), compression is the practical solution.

Open ShrinkPDF

Go to shrinkpdf.fyi. No account required, no file size limit, no upload to any server.

Select Maximum Compression

Maximum compression resamples embedded images to a lower DPI — effectively reducing the scan resolution after the fact. For a 300 DPI colour scan, Maximum compression produces output equivalent to approximately 72–96 DPI, reducing a 10MB scan to 1–2MB.

Verify the Output

Open the compressed PDF and check that text is legible at normal zoom. For portal submissions, the key check is whether IC numbers, account numbers, dates, and amounts are clearly readable — not whether images look perfect at 200% zoom.

Typical compression results for scanned documents

15MB colour scan (300 DPI) → 1–2MB after Maximum compression
8MB colour scan (300 DPI) → 600KB–1.2MB after Maximum compression
3MB grayscale scan (300 DPI) → 300–600KB after Maximum compression

Fix for Future Scans: Scan Settings to Use

The right scan settings produce portal-ready files directly, with no compression step needed:

Resolution: 150 DPI — sufficient for all screen-only submissions. Text is sharp at 100% zoom. In CamScanner: Settings → Scan Quality → Standard. Most apps don't expose DPI directly — "Standard" or "Normal" quality is approximately 150 DPI.
Colour mode: Grayscale — for payslips, transcripts, letters, forms. In iPhone Notes: use "Document" scan mode and it auto-optimises. In Microsoft Lens: select "Document" mode, not "Photo".
Export format: PDF directly — scan to PDF in one step. Don't scan to JPEG and then convert — this adds another encoding step and can increase file size.

Best Scanner App for Small File Sizes

Microsoft Lens (free, iOS and Android) consistently produces the smallest PDFs of any major scanner app. Reasons:

Better automatic edge detection and perspective correction — less background in the scan = less data
Good default compression without sacrificing text legibility
"Document" mode automatically applies higher contrast and noise reduction suitable for text documents
No ads on the free version, no upload limits

A document that produces a 3MB PDF in Adobe Scan typically produces 500KB–1MB in Microsoft Lens with equivalent visible quality.

Making a Scanned PDF Searchable (OCR)

Worth knowing separately from file size: a scanned PDF — and any PDF compressed by ShrinkPDF or a similar tool — is a page image, not real text. You can't select, copy, or Ctrl+F search any of it, even after compressing. If you need that, the fix is OCR (optical character recognition), which is a different process from compression and isn't something ShrinkPDF does.

If you regularly need to search through scanned documents — research papers, old records, archived contracts — dedicated OCR software like ABBYY FineReader converts scanned pages into searchable, selectable text with high accuracy across many languages. For a one-off document, some scanner apps (including Microsoft Lens) also offer basic built-in OCR for free, though accuracy is lower than dedicated software for dense or low-quality scans.

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Frequently Asked Questions

Why is my scanned PDF larger than the original printed document? ▼

The PDF file size has no relationship to the physical page count or density of the original document. What determines the PDF size is how many pixels were captured during scanning and how much compression was applied. A 300 DPI colour scan of a single typed A4 page stores millions of pixels — far more than the equivalent digital text document. The printed document 'contains' the same information in ink, which has no file size.

My scanner app doesn't let me change DPI. What can I do? ▼

Many phone scanner apps don't expose DPI settings directly. Look for quality settings labelled 'Standard/Normal/High/Ultra' — Standard or Normal is typically 150–200 DPI. If the app only has one quality level, try Microsoft Lens (free) which produces well-compressed output by default. Alternatively, compress the scan afterwards using ShrinkPDF's Maximum compression setting.

How much can I reduce a scanned PDF without making it unreadable? ▼

For text documents (payslips, letters, transcripts), Maximum compression in ShrinkPDF reduces file size by 70–85% while keeping text fully legible at normal screen zoom. This typically means going from 300 DPI colour to approximately 72–96 DPI, which looks sharp at 100% zoom. Text only starts to look degraded when zoomed in to 200%+ — which is not how official reviewers read submitted documents.

Is a heavily compressed scan acceptable for government submissions? ▼

Yes, for the vast majority of submissions. Government portal reviewers read documents on screen at standard zoom — they need to verify names, numbers, dates, and amounts, not inspect pixel quality. A Maximum-compressed scan remains perfectly legible for these purposes. The only cases where higher quality matters are signatures with very fine detail and certificates with intricate official seals that need to be examined closely.

Why is a phone camera photo larger than a scanner app scan? ▼

Phone camera photos are captured at the camera's native resolution (8–50 megapixels) without applying document-optimised compression. A scanner app like Microsoft Lens applies perspective correction, contrast enhancement, and targeted JPEG compression — producing a file specifically optimised for document readability at a much smaller size. Never embed raw phone camera photos in PDFs for submissions; always use a scanner app.