PDF defines 16 blend modes organised into four groups:

<strong>Normal group</strong> (no special compositing): <code>Normal</code> — The source colour replaces the backdrop (at full opacity) or blends linearly (at partial opacity). This is the default. <code>Compatible</code> — An alias for Normal, provided for backwards compatibility with PDF 1.3.

<strong>Darken group</strong> (result is darker than source or backdrop): <code>Multiply</code> — Multiplies source and backdrop colour values (0–1 range). Always equal to or darker than both. Equivalent to overlaying two projected transparencies. <code>Darken</code> — Keeps the darker of source or backdrop for each channel. <code>ColorBurn</code> — Darkens the backdrop to reflect the source by increasing contrast.

<strong>Lighten group</strong> (result is lighter than source or backdrop): <code>Screen</code> — Complements of source and backdrop are multiplied. Always equal to or lighter than both. Equivalent to projecting two images on a screen. <code>Lighten</code> — Keeps the lighter of source or backdrop for each channel. <code>ColorDodge</code> — Brightens the backdrop to reflect the source by decreasing contrast.

<strong>Contrast group</strong> (increases contrast): <code>Overlay</code> — Multiply for dark areas, Screen for light areas (based on backdrop luminosity). <code>SoftLight</code> — Similar to Overlay but softer; uses a complex formula for mid-tones. <code>HardLight</code> — Like Overlay but based on source luminosity rather than backdrop. <code>Difference</code> — Subtracts the darker from the lighter of source and backdrop. Produces black where identical, white where maximally different. Useful for spotting alignment differences. <code>Exclusion</code> — Lower contrast version of Difference.

<strong>HSL group</strong> (component-based compositing): <code>Hue</code> — Hue of source, saturation and luminosity of backdrop. <code>Saturation</code> — Saturation of source, hue and luminosity of backdrop. <code>Color</code> — Hue and saturation of source, luminosity of backdrop. Useful for colourising greyscale images. <code>Luminosity</code> — Luminosity of source, hue and saturation of backdrop. Inverse of Color.

Blend modes are set via the graphics state. In a PDF content stream, the graphics state is modified with the <code>gs</code> operator which takes an extended graphics state (ExtGState) dictionary name:

<code>/GS1 gs</code>

The named ExtGState dictionary (defined in the page's /Resources /ExtGState dictionary) contains the /BM entry:

<code>&lt;&lt; /Type /ExtGState /BM /Multiply /ca 0.5 &gt;&gt;</code>

Where <code>/ca</code> is the current non-stroking alpha (opacity). The blend mode applies to all subsequent painting operations until a new graphics state is set or the state is restored with <code>Q</code>.

<strong>Blend modes and transparency groups</strong> — Blend modes interact with transparency groups. A transparency group (a Form XObject with a /Group dictionary) creates an isolated compositing context. Within a group, blend modes composite objects against the group's internal backdrop (the group's contents so far) rather than the page backdrop. When the group is itself composited onto the page, the group's blend mode controls how the entire group composites against the page. This two-level compositing (within group, then group onto page) means the visual result of a blend mode depends on whether there is an enclosing transparency group. See <a href="/glossary/pdf-transparency-model" target="_blank" rel="noopener">PDF Transparency Model</a> for the full transparency group architecture.

In print production workflows (PDF/X documents), blend modes require careful handling:

<strong>Spot colour blending</strong> — Blend modes applied to spot colour channels can produce unexpected results because the compositing maths assumes additive or subtractive colour model arithmetic that does not directly apply to spot colours. Many PDF/X validators warn on blend modes applied to spot colour objects. See <a href="/glossary/pdf-spot-colour" target="_blank" rel="noopener">PDF Spot Colour</a> for spot colour context.

<strong>Overprint interaction</strong> — Blend modes interact with overprint settings. When overprint is enabled, the "knockout" model is replaced by an "overprint" model that composites the source over the backdrop using the normal blend mode — but if a different blend mode is also specified, the interaction can be complex and RIP-dependent.

<strong>Flattening for legacy RIPs</strong> — Older print RIPs that do not support PDF transparency natively require transparency flattening before output. Flattening converts blend mode transparency into rasterised regions that simulate the blending effect without transparency operators. The accuracy of flattened blend modes depends on the flattening quality setting — lower settings introduce visible approximations at blend boundaries.

The <a href="https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdf_reference_archive/blend_modes.pdf" target="_blank" rel="noopener">Adobe blend mode specification</a> provides the exact mathematical formulas for all 16 modes.