The size and quality of a scanned PDF are decided largely at scan time. Get the settings right and you'll rarely need heavy compression afterward.
Pick the right DPI
For documents, 150 DPI is usually plenty and keeps files small. 300 DPI is for high-quality printing. 600 DPI is overkill for everyday forms and produces huge files.
Grayscale unless colour matters
Scan in grayscale for text documents — smaller files, identical readability. Reserve colour for when it actually carries information.
Scan straight and well-lit
A straight, evenly lit scan compresses better and OCRs more accurately than a skewed, shadowed one.
Step by step
- Set the scanner to 300 DPI — enough for text and for OCR, without the bulk of 600.
- Choose grayscale unless the colour carries meaning.
- Square the page against the guide and make sure the light is even.
- Scan straight to PDF where the scanner offers it, rather than to images you combine later.
What usually goes wrong
600 DPI quadruples the size for no readable gain
Text does not become more legible above 300 DPI on a normal document. It just becomes a file nobody can upload.
Colour scans of black text waste most of the file
The scanner stores three channels to describe grey paper. Grayscale removes two of them.
Skew ruins OCR
A page a few degrees off straight measurably reduces recognition accuracy. Straightening at the glass is faster than fixing it later.
Frequently asked questions
What DPI should I scan at for an online form?
150 DPI grayscale is a good default — clear enough to read, small enough to upload.
What DPI should I scan documents at?
300 DPI. It is the standard for text, works well for OCR, and keeps file sizes manageable. 600 DPI is for photographs and fine detail.
Should I scan in colour or grayscale?
Grayscale unless the colour matters — it typically removes a third to a half of the file size with no loss of legibility.