Scanned PDF DPI Settings: The Complete Guide to Resolution
DPI — Dots Per Inch — is the single most important factor in determining your scanned PDF's file size. Scan at the wrong resolution and you end up with a 40 MB document that should be 5 MB, or a blurry unreadable mess that makes OCR impossible. Choosing the right DPI setting is the difference between a workflow that flows and one that constantly frustrates you. Most people scan at whatever the default setting is — often 300 DPI for document mode or 600 DPI for photo mode. For many use cases, 300 DPI is significantly higher than necessary, producing files 4× larger than needed. For others, 150 DPI is completely sufficient. And for archival or printing purposes, 600 DPI may actually be the right choice. This guide explains what DPI actually means for scanned documents, provides specific recommendations for common use cases (email attachments, OCR, archiving, printing), and shows you how to reduce file size after scanning if you're working with existing high-DPI scans. By the end, you'll have a clear mental model for choosing the right resolution every time.
What DPI Means for Scanned Documents
DPI (Dots Per Inch) describes how many individual pixel samples are captured per inch of the physical document. Higher DPI means more pixels, more detail, and larger file sizes. **The math is straightforward**: An A4 page is 8.27 × 11.69 inches. At 150 DPI, that's 1,240 × 1,753 pixels = 2.2 million pixels per page. At 300 DPI, that's 2,480 × 3,507 pixels = 8.7 million pixels — 4× more pixels, 4× more storage. At 600 DPI, you get 35 million pixels per page — 16× more than 150 DPI. The file size scales roughly proportionally: if a 150 DPI scan creates a 500 KB PDF page, a 600 DPI scan of the same page would be around 8 MB. The key insight is that human eyes have limits. Most people cannot distinguish a 300 DPI document from a 600 DPI document when reading text on a standard monitor or on paper. The extra pixels in a 600 DPI scan capture microscopic paper texture and ink variation that are invisible in normal use.
DPI Recommendations for Common Use Cases
Use this quick reference to choose the right DPI before scanning:
- 1Screen-only reading (emails, web portals, digital archives): 100–150 DPI — produces the smallest files, text is perfectly readable on any screen
- 2Standard printing (contracts, reports, invoices): 200–300 DPI — the professional standard, excellent print quality with reasonable file size
- 3OCR (text recognition): 300 DPI minimum — OCR accuracy drops significantly below 200 DPI; 300 DPI is the sweet spot for accuracy vs file size
- 4High-quality printing (brochures, photographs embedded in documents): 300–400 DPI — higher detail for graphics-heavy content
- 5Archival scanning (historical documents, legal records, artwork): 400–600 DPI — captures fine details for long-term preservation; accept the large file size as a trade-off
How to Check the DPI of an Existing Scanned PDF
If you received a scanned PDF or are working with existing files, you may need to determine what DPI they were scanned at before deciding whether to compress or resample. **On Windows**: Right-click the PDF, select Properties → Details (if available). Alternatively, open the PDF in Adobe Acrobat or a free alternative like PDF-XChange Viewer and check Document Properties → Description for image resolution information. **On Mac**: Open the PDF in Preview, go to Tools → Inspector → More Info. The DPI of the images inside the PDF is usually listed under 'Image DPI'. **Using the file size as a proxy**: If you don't have access to metadata, you can estimate DPI from file size. For a single-page A4 scanned document: under 200 KB suggests 72–100 DPI; 200–500 KB suggests 150 DPI; 500 KB–2 MB suggests 200–300 DPI; 2 MB–5 MB suggests 300–400 DPI; over 5 MB per page suggests 600+ DPI or very high-quality settings. **The practical implication**: If your existing scanned PDF is 5+ MB per page, it was almost certainly scanned at 400–600 DPI. For most uses, it can be compressed aggressively without any perceptible quality loss.
Reducing File Size for High-DPI Scanned PDFs
If you have existing high-DPI scans that are too large to use comfortably, compression is the most practical solution. LazyPDF's compression tool handles this intelligently — it re-encodes the images inside the PDF at a resolution appropriate for the intended use without requiring you to rescan. For a 600 DPI scanned document intended for screen reading, compression can reduce file size by 80–90% with no visible difference on screen. The human eye simply cannot perceive the difference between 600 DPI and 150 DPI when viewing on a standard monitor. For documents you'll print, compression to a 200–300 DPI equivalent is safe. You'll get excellent print quality with files 70–80% smaller than the original 600 DPI scan. If you're managing a large archive of scanned documents, developing a consistent DPI policy saves storage and time. Establish a standard like '300 DPI for all archival scans, compressed to 150 DPI equivalent for working copies' so you always have a high-quality master and a practical working version.
Frequently Asked Questions
What DPI should I scan documents for email attachments?
100–150 DPI is ideal for documents you'll send as email attachments. At 150 DPI, text is perfectly readable on any screen and the file size is minimal — a 5-page document typically comes in under 1 MB. Most email services have 20–25 MB attachment limits, but staying under 5 MB is good practice to avoid issues with stricter recipient mail servers.
Does scanning at higher DPI always produce better OCR results?
Up to 300 DPI, higher resolution improves OCR accuracy because the engine can distinguish characters more clearly. Above 300 DPI, accuracy improvements are marginal for most printed text. The sweet spot for OCR is 300 DPI — it gives excellent recognition accuracy without the bloated file sizes of 400–600 DPI scans.
My scanner only offers 75, 150, 300, or 600 DPI. Which should I choose?
For everyday document scanning (invoices, contracts, forms), choose 300 DPI as your default. For quick scans you'll only view on screen and share via email, choose 150 DPI. Only use 600 DPI for archival documents where you need to capture fine details, historical records, or artwork. The 75 DPI setting is rarely useful — it's too low even for comfortable screen reading of text-heavy documents.
Can I change the DPI of an already-scanned PDF?
You cannot increase the effective DPI of an already-scanned PDF (you can't recover detail that wasn't captured). You can reduce effective DPI through compression, which re-encodes images at a lower resolution. If you need a higher resolution version of an existing low-quality scan, you must rescan the original document.