A bad scan is worse than no scan — a document that cannot be read is useless, and a crooked or shadowed scan looks unprofessional when submitted to a government office, employer, or bank. These problems are entirely preventable. Here is how to get clean, straight, legible scans every time.
Problem 1: Crooked or tilted pages
The most common scan problem. Even a 2–3 degree tilt makes a document look careless and can cut off text at the edges when zoomed.
Prevention:
- Place the document squarely on the surface with the top edge parallel to the edge of the table.
- Use a document scanner or the Notes/Files app on iPhone — these automatically detect the page edges and apply perspective correction and deskewing.
- On Android, Google Drive's scanner also auto-deskews. If the preview looks tilted, tap to retake rather than accepting a bad scan.
Fix after the fact: Use Filero's Rotate PDF tool for 90-degree rotations (e.g. a landscape page scanned as portrait). For small tilts, a dedicated image editor (GIMP, Photoshop) can straighten the image before you convert it to PDF.
Problem 2: Dark edges and shadows
Dark shadows at the edges or gutter of a scanned page — especially for bound books or documents with a curve — are caused by the page not lying perfectly flat, so light does not reach the edges evenly.
Prevention:
- Use a flatbed scanner with the lid pressed firmly down. For thick books, place a heavy flat object on top of the open lid to press the pages flat.
- Scan near a bright window or under a ceiling light for phone scans — even lighting reduces shadows dramatically.
- Avoid direct flash — it creates glare in the centre and dark edges. Use ambient light instead.
Fix after the fact: In a photo editor, use the Levels or Curves tool to brighten the shadows, or the Clone Stamp to paint over edge shadows. iPhone's Markup tool has basic brightness and contrast controls.
Problem 3: Blurry or out-of-focus pages
Phone camera scans can be blurry when the phone moved during capture, the lighting was insufficient, or the camera could not focus at close range.
Prevention:
- Hold the phone still for at least one second after the auto-focus locks. The Notes scanner on iPhone does not capture until focus is confirmed — let it do its job.
- Use good lighting. Low light forces the camera to use a slower shutter speed, making motion blur more likely.
- Tap the screen to manually focus on the document if the camera is hunting.
- Keep the phone at least 20–30 cm above the document — too close can exceed the minimum focus distance.
Problem 4: Grey, muddy, or low-contrast text
Text that should be crisp black appears grey or washed out. This is usually caused by scanning a photocopy (where the originals were already grey), too-low DPI, or the scanning app applying too much compression.
Prevention:
- Scan at 300 DPI minimum for documents with text.
- Use your scanner's "Document" or "Black and White" mode for text-only documents — this applies a threshold filter that converts grey pixels to pure black or pure white, dramatically improving legibility.
- Avoid JPEG compression for scanned documents. JPEG introduces artefacts that blur text edges. Save as PNG first if your scanner allows, then convert to PDF using Filero's Image to PDF tool.
Problem 5: Pages scanned in the wrong order
Multi-page documents often end up with pages out of sequence, especially when scanning one side at a time on a single-sided scanner.
Fix: Use Filero's Merge PDF tool to reorder pages. Upload each page as a separate file, then drag them into the correct order before combining. Or use the Split PDF tool to extract and reorder pages from a single scan.
The best phone scanning apps (ranked)
- iPhone Notes / Files: Best for everyday documents. Auto-deskews, auto-crops, saves directly to PDF. No extra app needed.
- Google Drive (Android/iPhone): Very good deskewing and perspective correction. Saves to Drive automatically. Free.
- Microsoft Lens: Available on both platforms. Excellent for whiteboards and business cards in addition to documents. Free.
- Adobe Scan: Excellent OCR that makes scans searchable. Free with an Adobe account.
Frequently asked questions
Does it matter if I scan in colour or black and white?
For text-only documents, black and white scanning produces sharper, smaller files — the threshold filter eliminates grey noise and creates pure black text. For documents with colour diagrams, photos, or logos, scan in colour. Never scan a full-colour document in black and white if the colour carries meaning.
What DPI should I use for different document types?
Text documents: 300 DPI. Photos and artwork: 300–600 DPI. Technical drawings with fine lines: 600 DPI. Documents that will only ever be viewed on screen: 150 DPI is sufficient and keeps file sizes smaller.
Ready to try it?
Use Filero's free Image to PDF tool. No account needed, works on any device.
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