Tenger Plugin: pdf watermark
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PDF Ink Lite – Free PDF Watermark & Password Protection
Watermark and password protect PDFs with WooCommerce, EDD, and Download Monitor. No Ghostscript required, PHP 8+ compatible. Since 2014 (FKA WaterWoo)
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RayEtun Media Protection – Watermark and File Access Control
Watermark images and PDFs, protect downloadable files, and control who can access them — in one plugin, on standard WordPress APIs.