How to Change Fonts in PDFs Without Breaking the Format
The Problem
Swapping fonts in a PDF sounds simple, but it usually destroys the document.
Most tools either can't change fonts at all, or they produce garbled output with shifted text, broken spacing, and overlapping characters.
The core issue is that PDFs don't treat fonts like word processors do. Instead of storing "use Arial at 12pt," they embed:
- Subset fonts containing only the glyphs actually used
- Per-character positioning with custom kerning pairs
- Transformation matrices that scale and rotate text
Simply swapping the font reference breaks everything because the new font has different glyph widths, different kerning tables, and different metrics.
Technical Explanation
- Each character has explicit x,y coordinates - there's no automatic flow
- Glyph widths vary between fonts - "W" in Arial is not the same width as "W" in Times
- Kerning pairs are font-specific - "AV" spacing in one font differs from another
- Embedded subset fonts only contain used glyphs - you can't just reference a new font
- Baseline positions, ascenders, and descenders differ between font families
- Line heights and paragraph spacing are baked in as absolute positions
Source PDF
API Calls
Full Code
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How PDFDancer Solves This
- Analyzes the original font's metrics - widths, kerning, baseline, x-height
- Recalculates every character position for the new font
- Preserves visual alignment - text stays where it should be
- Handles line reflow when text expands or contracts
- Embeds only the required glyphs from the new font (proper subsetting)
- Maintains paragraph boundaries - no text bleeding into margins
- Supports mixing fonts - change headers without touching body text
- Works with OFL (Open Font License) fonts included in PDFDancer