Most people scan a document once, get a 40 MB file, and assume that is just how scanning works. It is not. Two settings decide almost everything about the result: the resolution and the color mode. Get those right and a ten-page contract comes out crisp at a few hundred kilobytes. Get them wrong and you either squint at blurry text or fight an email that will not send.
The two settings that matter most
Every scanner, from a flatbed in an office to a phone app, exposes the same two controls under different names. Resolution (measured in DPI, dots per inch) says how many pixels the scanner captures per inch of paper. Color mode says how much color information it keeps for each of those pixels.
Together they set the raw amount of data. A single A4 page at 300 DPI is roughly 2,480 by 3,508 pixels. In full color, that is about 26 MB before any compression. In black and white, each pixel needs a single bit, so the same page is closer to 1 MB uncompressed, and far smaller once compressed. That difference is why the same stack of paper can become a 3 MB file or a 60 MB file depending on a drop-down you may never have opened.
Resolution: how many DPI do you really need?
More DPI means a sharper image and a bigger file, but the benefit stops somewhere. Text that is already readable at 300 DPI does not become more readable at 1200 DPI. It only becomes heavier.
- 150 DPI: fine for reading on a screen. Small print can look soft, and it is not ideal for OCR.
- 200 DPI: a good economy setting for long documents where file size matters more than perfection.
- 300 DPI: the standard for text documents, forms and contracts. Reliable for printing and for text recognition.
- 400 to 600 DPI: useful for tiny text, fine line drawings, small photographs or documents you plan to archive.
- 1200 DPI and above: almost never worth it for paper documents. Reserve it for film, slides or very small originals.
If you are unsure, scan one page at 200 DPI and one at 300 DPI, zoom in to 200 percent on screen and compare. If you cannot tell them apart, use the lower setting.
Color, grayscale or black and white?
Black and white (also called bitonal or line art)
Every pixel is either black or white. This is the smallest option and works beautifully for printed text, typed letters and forms. It struggles with pencil, light grey stamps, highlighter and photographs, which can turn into harsh blobs or disappear.
Grayscale
Shades of grey, no color. A safe middle ground for documents with handwriting, pencil notes, coffee stains or fine detail. Files are larger than black and white but much smaller than color.
Color
Choose color when color carries meaning: a signed page with blue ink, a colored chart, a stamp, a photograph, a children's drawing. For a plain black-on-white letter, color just adds weight and slightly dirties the background.
A practical rule: start with black and white at 300 DPI for text. If anything on the preview looks wrong, step up to grayscale. Only go to color if you can name a reason.
Compression and file formats inside the scan
A PDF is a container. Inside it, each scanned page is usually stored as an image, and how that image is compressed changes the file size dramatically.
- JPEG compression suits color and grayscale pages. Quality settings between roughly 70 and 85 percent usually look the same as the original to the eye while cutting size sharply.
- Lossless or bitonal compression suits black and white text. Schemes made for this are very efficient, so a page can be tiny.
- TIFF files are an older scanner output. Large but fine as a master copy. Convert to PDF for sharing.
If your scanning software offers a choice such as "High quality" versus "Small size", the second is often the better default for documents you only need to read, send or file.
Searchable PDF or image-only PDF?
An ordinary scan is a photograph of a page. You cannot select, search or copy the words, and screen readers cannot read them. If your scanner or app offers OCR (optical character recognition), it adds an invisible layer of text on top of the image so the PDF becomes searchable.
OCR works best on clean 300 DPI scans with straight pages and good contrast. If recognition looks poor, the cause is usually the scan, not the OCR: skewed pages, shadows near the spine, or a resolution below 200 DPI. Rescanning a few pages at 300 DPI often fixes more than switching software.
Our tools do not perform OCR. They work with the PDF you already have, so run OCR in your scanner software first if you need searchable text. If you only need the existing text out of a normal PDF, PDF to Text does that.
Three setups that work for most people
- Everyday paperwork (letters, forms, invoices): 300 DPI, black and white, searchable if available. Expect well under a megabyte per page.
- Receipts and thin paper: 300 DPI, grayscale. The extra tones keep faded thermal print readable.
- Photos, brochures, anything colorful: 300 to 400 DPI, color, medium JPEG quality. Check the result at full size before you shred the original.
For books and documents you want to keep for years, scan at 400 to 600 DPI in grayscale or color, keep the original master, and make a smaller copy for everyday use.
Fix problems after scanning
Scanners are not always perfect, and that is fine. Most defects can be repaired afterwards without rescanning:
- Sideways or upside-down pages: rotate them permanently with Rotate PDF.
- Black borders or wide margins: trim every page at once with Crop PDF.
- File too big to send: shrink it with Compress PDF. Scans compress especially well, often by half or more.
- Blank back sides: remove them with Remove Pages.
Do these in that order: rotate, crop, remove, then compress last. Compressing first and editing afterwards means working on a lower-quality copy.
Scanning with a phone
Phone scanning apps are convenient and often good enough, but the same rules apply. Put the page on a plain, dark surface in even light, hold the phone parallel to the paper, and avoid casting your own shadow across the page. Most apps correct perspective automatically; let them. If the app offers a quality or size setting, choose medium or standard for documents and the highest setting only for photographs.
