PDF ： Kindle、Kobo ，

Why PDFs Look Bad on e-Paper

E-ink screens render a handful of gray levels and refresh slowly. A scanned page with a cream or gray paper background becomes a muddy rectangle; faint pencil or thin pen strokes drop below the threshold the display can show; and JPEG-style compression noise turns into visible dithering. The file that looked fine on a monitor is barely readable on a Kindle.

The fix is to decide, per pixel, whether it's ink or paper — and that's what this tool does. Otsu thresholding analyzes each page's brightness histogram and picks the split that best separates text from background, then bleaches everything above it to pure white and pushes everything below it to black.

The Optimization Pipeline

Four stages, each tuned for monochrome e-paper.

Binarize or Grayscale

Binarize mode is the aggressive option: pure black text on pure white, the cleanest possible result for text-heavy documents and the smallest output. Grayscale mode keeps shading — photos, diagrams, pencil texture — but stretches the histogram so the paper tint lifts toward white and ink sinks toward black.

Vector PDFs work too. Pages are rasterized at high resolution in the background, processed, and recompiled, so a born-digital document gets the same e-ink treatment as a scan.

How It Compares

Grayscale conversion and brightness sliders can't isolate ink from paper the way histogram thresholding does.

Your Documents Stay on Your Device

Manuscripts, exam sheets, and scanned contracts are exactly the files you don't want on a stranger's server. The whole pipeline — histogram analysis, thresholding, dilation, recompression — runs in your browser, and closing the tab wipes everything.