How to Scan Documents to PDF with Your Phone: Light, Angle and File Size

To scan a document to PDF with a phone, open a scanner in an app, let it outline the page, take the photo, adjust the crop and filter, add any further pages and save as PDF. A phone scan is really a photograph that software turns into a page, so the quality of that photograph shapes the result. This guide covers how phone scanning works, how to get a clean capture, and how to keep the resulting PDF a sensible size.

What a phone scanner does

Google's help for the scanner built into the Google Drive app for Android shows the steps in one scanner; other apps label their buttons differently. You point the camera at the document, a blue line outlines it to show where the picture will be cropped, and you take the photo, manually or with automatic capture. Then you adjust: crop and rotate, apply a filter for colour or greyscale, clean away stains or fingers, add more pages or retake one, and save as PDF or JPG.

If the scan is later read by text-recognition software, Wikipedia's article on optical character recognition describes pre-processing steps such as de-skewing the page so lines of text run straight, despeckling spots and binarisation, converting the image to black and white to separate text from background. A straight, clean capture gives those steps less to correct.

Getting a clean capture

The page for our scanner app (below) notes that lighting and focus affect the result. These habits help with any phone scanner:

1. Light: bright, even light without harsh shadows. Avoid standing between the light and the page, where your own shadow falls across it.
2. Flatness: lay the page flat on a contrasting surface, such as white paper on a dark table, so its edges stand out.
3. Angle: hold the phone parallel to the page, directly above it. A tilted phone produces a trapezoid the software must stretch back.
4. Steadiness: rest your elbows on the table, let the camera focus, and use automatic capture if the app offers it.
5. Glare: on glossy paper or receipts, tilt the light source rather than the phone to move reflections off the text.

A worked example: megapixels to page resolution

Wikipedia's dots-per-inch article explains that pixels per inch (PPI) describes the input resolution of a photograph, and for a scanned image relates its pixels to the size of the original. A 12-megapixel camera shooting in a 4:3 frame takes photos of about 4,000 × 3,000 pixels. If a letter-size page, 8.5 × 11 inches, fills the frame, its 11-inch length spans about 4,000 pixels: about 364 pixels per inch. Its 8.5-inch width spans about 3,000 pixels: about 353 pixels per inch. Because the page shape is not exactly the frame shape, the narrower figure is the one that limits you: roughly 350 pixels per inch.

The catch is that a page spanning only half the frame's height has half the linear resolution, about 180 pixels per inch. Move closer, so the page fills most of the frame, rather than zooming digitally.

Filters and colour

Choose the filter for the content:

• Black and white or high-contrast modes suit typed text on a plain background.
• Greyscale suits pencil notes and documents with shading.
• Colour is worth it only when colour carries meaning: stamps, highlighted passages, photos or coloured forms.