Rendering

Epistola uses iText Core 9.5.0 (Kotlin) as its primary rendering engine, producing PDF documents directly from the template’s node/slot graph.

Primary engine: iText

The iText engine maps the template’s internal node/slot graph directly to iText layout elements. Key characteristics:

Fast — Typical render times of 10–200ms per document
Pure JVM — No external processes or dependencies required
Direct mapping — Template nodes convert directly to iText elements without an intermediate representation

Rendering features

Automatic page headers and footers — Rendered via iText event handlers, repeating on every page
Automatic pagination — Content flows across pages with configurable spacing between blocks
Cascade-resolved styles — The full style cascade (theme → template → preset → inline) is resolved at render time
Server-side expression evaluation — All expressions are evaluated on the server before rendering

PDF/A-2b compliance

PDF/A compliance can be toggled per template in the settings tab. When enabled, the engine produces documents conforming to the PDF/A-2b standard — embedding all fonts, avoiding external dependencies, and including required metadata for long-term archival.

Fallback: Playwright + HTML

For layouts requiring complex CSS that the iText engine does not support, a fallback path uses Playwright to render HTML to PDF. This is slower but handles edge cases involving advanced CSS layouts.

Output formats

PDF — The default output format, used for production documents
HTML — Available for email body generation and web-based previews