DPI / PPI

DPI (dots per inch), sometimes called PPI for pixels per inch, measures how densely packed an image's detail is, how many dots fit into each inch when it is printed or scanned. It is the number that decides whether an image looks sharp or soft at a given size.

The right DPI depends on the job. Around 72 to 96 DPI is plenty for something only ever viewed on screen. For printing, 300 DPI is the common standard, and OCR also reads best at roughly 300 DPI, enough detail to recognize characters cleanly. Scanning below that often makes text fuzzy and hurts recognition accuracy.

DPI cuts both ways in PDFs. Higher DPI means sharper images but bigger files, so a scan-heavy document can become enormous. This is why compression often downsamples images to a sensible DPI: high enough to look good, low enough to keep the file manageable.