Patches poppler's pdftocairo to be able to set zlib compression level when generating png files. Tested with pdftocairo 0.22.5.

pdftocairo-pngcompression.patch

diff --git a/goo/PNGWriter.cc b/goo/PNGWriter.cc
index b775600..d484991 100644
--- a/goo/PNGWriter.cc
+++ b/goo/PNGWriter.cc
@@ -35,8 +35,20 @@ struct PNGWriterPrivate {
   int icc_data_size;
   char *icc_name;
   bool sRGB_profile;
+  int zlib_compression_level;
 };
 
+PNGWriter::PNGWriter(int zlib_compression_level, Format formatA)
+{
+  priv = new PNGWriterPrivate;
+  priv->format = formatA;
+  priv->icc_data = NULL;
+  priv->icc_data_size = 0;
+  priv->icc_name = NULL;
+  priv->sRGB_profile = false;
+  priv->zlib_compression_level = zlib_compression_level;
+}
+
 PNGWriter::PNGWriter(Format formatA)
 {
   priv = new PNGWriterPrivate;
@@ -45,6 +57,7 @@ PNGWriter::PNGWriter(Format formatA)
   priv->icc_data_size = 0;
   priv->icc_name = NULL;
   priv->sRGB_profile = false;
+  priv->zlib_compression_level = Z_BEST_COMPRESSION;
 }
 
 PNGWriter::~PNGWriter()
@@ -107,7 +120,7 @@ bool PNGWriter::init(FILE *f, int width, int height, int hDPI, int vDPI)
   }
 
   // Set up the type of PNG image and the compression level
-  png_set_compression_level(priv->png_ptr, Z_BEST_COMPRESSION);
+  png_set_compression_level(priv->png_ptr, priv->zlib_compression_level);
 
   // Silence silly gcc
   png_byte bit_depth = -1;
diff --git a/goo/PNGWriter.h b/goo/PNGWriter.h
index c73c964..9bfcdd2 100644
--- a/goo/PNGWriter.h
+++ b/goo/PNGWriter.h
@@ -35,6 +35,7 @@ public:
    */
   enum Format { RGB, RGBA, GRAY, MONOCHROME };
 
+  PNGWriter(int compression_level, Format format = RGB);
   PNGWriter(Format format = RGB);
   ~PNGWriter();
 
diff --git a/utils/pdftocairo.cc b/utils/pdftocairo.cc
index 2fa0af7..24faab3 100644
--- a/utils/pdftocairo.cc
+++ b/utils/pdftocairo.cc
@@ -100,6 +100,7 @@ static GBool gray = gFalse;
 static GBool transp = gFalse;
 static GooString icc;
 static int jpegQuality = -1;
+static int pngCompression = 9;
 
 static GBool level2 = gFalse;
 static GBool level3 = gFalse;
@@ -124,6 +125,8 @@ static const ArgDesc argDesc[] = {
 #if ENABLE_LIBPNG
   {"-png",    argFlag,     &png,           0,
    "generate a PNG file"},
+  {"-pngcompression", argInt, &pngCompression, 0,
+   "zlib compression level to use when generating PNG files (0 - 9)"},
 #endif
 #if ENABLE_LIBJPEG
   {"-jpeg",   argFlag,     &jpeg,           0,
@@ -265,13 +268,13 @@ void writePageImage(GooString *filename)
   if (png) {
 #if ENABLE_LIBPNG
     if (transp)
-      writer = new PNGWriter(PNGWriter::RGBA);
+      writer = new PNGWriter(pngCompression, PNGWriter::RGBA);
     else if (gray)
-      writer = new PNGWriter(PNGWriter::GRAY);
+      writer = new PNGWriter(pngCompression, PNGWriter::GRAY);
     else if (mono)
-      writer = new PNGWriter(PNGWriter::MONOCHROME);
+      writer = new PNGWriter(pngCompression, PNGWriter::MONOCHROME);
     else
-      writer = new PNGWriter(PNGWriter::RGB);
+      writer = new PNGWriter(pngCompression, PNGWriter::RGB);
 
 #if USE_CMS
 #ifdef USE_LCMS1