diff --git a/src/ocrmypdf/leptonica.py b/src/ocrmypdf/leptonica.py
index 50fdb3b1..9f7e4b40 100644
--- a/src/ocrmypdf/leptonica.py
+++ b/src/ocrmypdf/leptonica.py
@@ -496,6 +496,9 @@ class Pix:
             raise LeptonicaError("Generate PDF data failed")
         return CompressedData(p_compdata[0])
 
+    def invert(self):
+        return Pix(lept.pixInvert(ffi.NULL, self._pix))
+
     @staticmethod
     def _pix_destroy(pix):
         p_pix = ffi.new('PIX **', pix)
diff --git a/src/ocrmypdf/optimize.py b/src/ocrmypdf/optimize.py
index 916e92c6..4af75f16 100644
--- a/src/ocrmypdf/optimize.py
+++ b/src/ocrmypdf/optimize.py
@@ -261,6 +261,8 @@ def transcode_pngs(pike, pngs, root, log, options):
         # Open, transcode (!), package for PDF
         try:
             pix = leptonica.Pix.open(png_name(root, xref))
+            if pix.depth == 1:
+                pix = pix.invert()  # PDF assumes 1 is black for monochrome
             compdata = pix.generate_pdf_ci_data(
                 leptonica.lept.L_FLATE_ENCODE, 0
             )