diff --git a/pkgs/by-name/pa/paperless-ngx/package.nix b/pkgs/by-name/pa/paperless-ngx/package.nix
index 3d3c8a0afdd1..98040c2ebcb2 100644
--- a/pkgs/by-name/pa/paperless-ngx/package.nix
+++ b/pkgs/by-name/pa/paperless-ngx/package.nix
@@ -315,6 +315,10 @@ python.pkgs.buildPythonApplication rec {
     export HOME=$(mktemp -d)
     export XDG_DATA_DIRS="${liberation_ttf}/share:$XDG_DATA_DIRS"
     export PAPERLESS_NLTK_DIR=${passthru.nltkDataDir}
+    # Limit threads per worker based on NIX_BUILD_CORES, capped at 256
+    # ocrmypdf has an internal limit of 256 jobs and will fail with more:
+    # https://github.com/ocrmypdf/OCRmyPDF/blob/66308c281306302fac3470f587814c3b212d0c40/src/ocrmypdf/cli.py#L234
+    export PAPERLESS_THREADS_PER_WORKER=$(( NIX_BUILD_CORES > 256 ? 256 : NIX_BUILD_CORES ))
   '';
 
   disabledTests = [