PDF glossary
Text render mode
A setting that says how glyphs are painted: filled, stroked, both, invisible, or used as a clipping path.
Mode 0 fills glyphs, the normal case. Mode 1 strokes outlines, mode 2 does both, and mode 3 draws nothing visible at all.
Invisible text (mode 3) is how OCR software places a searchable layer over a scanned image.
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