Compare two PDF drawings page-by-page. Every pixel is color-coded to show exactly what changed.
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100% local processing. Your PDFs are never uploaded anywhere. All comparison happens inside your browser tab — safe for proprietary and CUI documents.
1
Load your PDFs
Drag and drop — or click to browse — into the Original PDF and Revised PDF slots in the sidebar. Multi-page PDFs are fully supported; all pages are compared in order (page 1 vs 1, page 2 vs 2, etc.).
2
Configure settings (optional)
Adjust Render Scale, Diff Threshold, and Background mode if needed. Default settings work well for most drawings. Use the 🎨 button next to the legend to customise overlay colors. See the Settings tab for full details.
3
Run Compare
Click ▶ Run Compare. Each page is rendered and compared pixel-by-pixel. A progress bar shows status for multi-page documents. Pages are processed in order — page 1 vs 1, page 2 vs 2, etc.
4
Review results
The overlay appears in the main canvas. Use the ‹ › page navigator to flip between pages. The status bar at the bottom shows pixel counts and % difference for the current page.
5
Navigate the canvas
− / + buttons adjust zoom in 25% steps · FIT fits the full page · W fits width · H fits height
Alt + Scroll zooms toward your mouse without affecting browser zoom
Right-click drag or Middle mouse drag to pan in any direction
6
Region Inspector
Hold Ctrl and drag a rectangle over any area of the overlay. A popup appears showing three panels side-by-side: the color-coded overlay, the original, and the revised — all cropped to your selection at full resolution.
Use ⎘ Copy to copy the composite to your clipboard (paste directly into email, Word, Teams) or ⬇ JPG to save it. The popup is draggable. Click outside or press Escape to close.
7
Export
Set a Filename Prefix in the Export section (default: drawdiff) — pages save as prefix_001.jpg, prefix_002.jpg, etc.
⬇ JPGs saves each page as a separate JPEG. ⬇ PDF packages all comparison pages into a single PDF document.
Color Code (customisable via 🎨)
Green — content present in the Original only (removed or moved in Revised)
Red — content present in the Revised only (added since Original)
Cyan — content present in both, within the Diff Threshold (unchanged)
Dark / White — background (empty space), color depends on Background setting
💡 Tip: If the result looks mostly green and red with very little cyan, the drawings are probably shifted relative to each other. Use the Alignment Tool instead.
Use this when a view has shifted position between revisions, making the standard compare too noisy to be useful. You crop a region on each drawing, pick a matching reference point, and DrawDiff shifts one over the other before comparing.
1
Open the Alignment Tool
Click ⊕ Alignment Tool in the sidebar. Both PDFs load side-by-side at full render resolution — the panes auto-fit on open. Both PDFs must be loaded first; running a standard compare first is not required.
2
Navigate the panes
Each pane has independent zoom controls in its header: − + FIT buttons, Alt + Scroll to zoom toward the mouse, and Right-click drag or Middle mouse drag to pan.
Zoom in tight on the area you want to compare — the panes render at full resolution so quality stays sharp at any zoom level.
3
Crop-select a region on each drawing
Click and drag to draw a crop rectangle on the Original pane, then do the same on the Revised pane. The selection box stays in the correct position as you zoom. Select the same general area on both — it doesn't need to be pixel-perfect.
4
Set reference alignment points
Once both regions are selected, hover over a distinctive feature — a sharp corner, line endpoint, or box intersection — on the Original pane. A magnifier loupe shows 8× zoom with a crosshair and live pixel coordinates. Click to place the reference point, then do the same on the identical feature in the Revised pane.
For precision: Move your mouse to roughly the right spot, then use Arrow keys to nudge exactly 1 full-res pixel at a time. Shift + Arrow nudges 0.1px for sub-pixel precision. The loupe updates live as you nudge. Press Space or Enter to confirm.
Right-click on a pane to clear and re-pick that side's reference point.
Best reference features: Sharp corners of boxes or borders, line endpoints, and clear intersections. Avoid curves, text edges, or anti-aliased areas — these have no single definitive pixel.
5
Run Aligned Compare
Click ▶ Run Aligned Compare. DrawDiff shifts the Revised crop so the two reference points align exactly (using bilinear interpolation for sub-pixel accuracy), then runs the pixel comparison. The result opens in a popup automatically and a thumbnail appears in the bottom bar.
6
Review result — zoom, pan, inspect
The result popup supports full zoom and pan: − + FIT buttons, Alt + Scroll, and Right-click drag / Middle mouse drag to pan. The popup is draggable by its header. Close it with ✕ or Escape — the thumbnail in the bottom bar lets you reopen it anytime.
7
Fine-tune the alignment
If the result shows thin green/red fringing on lines that should be cyan — a sign of residual sub-pixel misalignment — use the ⊕ FINE-TUNE OFFSET strip at the bottom of the result popup.
Drag the X and Y sliders (±10px in 0.1px steps) to nudge the alignment. The compare re-runs live as you drag with bilinear interpolation applied, so you see the fringing disappear in real time. The base shift from your reference points is shown for reference.
Click ↺ Reset offset to snap back to the original ref-point alignment if the fine-tune isn't helping.
8
Save or reset
⬇ Save JPG and ⎘ Copy are available in both the popup header and the bottom bar — they export the current result including any fine-tune offset applied. Click ↺ Reset to clear all selections and start fresh.
💡 When to use fine-tune: If you see thin 1-2px green/red fringes on lines that should be identical, that's sub-pixel misalignment — either from a fractional PDF shift or a slightly imprecise ref point click. Nudge Y by ±0.5px first (vertical fringing is most common), then X if needed. The fringing should collapse to cyan when you hit the right offset.
All settings are in the sidebar. Changes to Render Scale, Diff Threshold, and Background take effect the next time you click Run Compare. Colour settings take effect immediately on re-run.
Render Scale
Number 0.5–8 · Default: 3
Controls how large the PDF is rasterized before comparison. Scale 3 renders at 216 DPI (3× the PDF's native 72 DPI). Higher values capture finer detail but increase processing time and memory.
At 0.5–1: fast, low-res — good for quick checks on simple documents.
At 3 (default): good balance of detail and speed for most engineering drawings.
At 5–6: high detail for drawings with fine lines, tight tolerances, or dense annotations.
At 7–8: maximum detail — use for critical reviews. May be slow on large multi-page documents.
Also applies to the Alignment Tool — higher scale means more pixels to pick your ref point from, which improves precision.
Recommended: 3 general use · 5–6 detailed drawings · 0.5–1 quick checks
Diff Threshold
Slider 0–128 · Default: 32
The maximum average color difference between two pixels before they're counted as "different." PDF renderers apply anti-aliasing, so the same line can produce slightly different pixel values between renders even when nothing changed. The threshold absorbs this noise.
At 0: any pixel difference is flagged — maximally sensitive, expect false positives on edges.
At 32 (default): minor rendering noise is ignored, genuine changes are caught.
At 128+: only large color shifts are flagged — may miss subtle changes.
Recommended: 32 general use · 0–10 catch every pixel · 60+ major changes only
Background
Dropdown · Dark / White / Original
Controls the color of empty (background) space in the overlay.
Dark: Near-black background. Focuses attention on the colored diff areas.
White: White background, matching how drawings look on paper. Easier to read as a document.
Original: Background pixels taken from the Original PDF. Shows spatial context so you can see where changes sit relative to surrounding content.
Filename Prefix
Text input · Default: drawdiff
The prefix used when exporting JPG files. Pages save as prefix_001.jpg, prefix_002.jpg, etc. Enter your drawing number or project code here before exporting — e.g. DWG-4521 produces DWG-4521_001.jpg. Invalid filename characters are automatically replaced with underscores.
Colour Settings 🎨
Click the 🎨 button next to the Color Legend
Opens the colour customisation panel. Change the overlay colors for Original Only, New Only, and Unchanged content — with opacity control for each.
Presets:
· Neon — default green/red/cyan, high contrast on dark backgrounds
· Print — red/teal/grey, better for printing or projecting
· Accessible — orange/blue/grey, safe for deuteranopia and protanopia
A live preview canvas shows how the three colors look before you apply. Changes re-run the compare automatically.
💡 Workflow tip: Start at Render Scale 3. If you see excessive green/red noise on lines that look identical, raise the Diff Threshold. If you think real changes are being missed, lower it or raise Render Scale. If the whole result looks like noise (everything green/red), the drawings are shifted — use the Alignment Tool.