Patches poppler's pdftocairo to be able to set jpeg quality parameter. Tested with pdftocairo 0.22.5.

pdftocairo-jpegquality.patch

diff --git a/utils/pdftocairo.cc b/utils/pdftocairo.cc
index bf7072c..743016d 100644
--- a/utils/pdftocairo.cc
+++ b/utils/pdftocairo.cc
@@ -99,6 +99,7 @@ static GBool mono = gFalse;
 static GBool gray = gFalse;
 static GBool transp = gFalse;
 static GooString icc;
+static int jpegQuality = -1;
 
 static GBool level2 = gFalse;
 static GBool level3 = gFalse;
@@ -127,6 +128,8 @@ static const ArgDesc argDesc[] = {
 #if ENABLE_LIBJPEG
   {"-jpeg",   argFlag,     &jpeg,           0,
    "generate a JPEG file"},
+  {"-jpegquality", argInt, &jpegQuality,    0,
+   "set JPEG quality (0 - 100)"},
 #endif
 #if ENABLE_LIBTIFF
   {"-tiff",    argFlag,     &tiff,           0,
@@ -293,9 +296,9 @@ void writePageImage(GooString *filename)
   } else if (jpeg) {
 #if ENABLE_LIBJPEG
     if (gray)
-      writer = new JpegWriter(JpegWriter::GRAY);
+      writer = new JpegWriter(jpegQuality, false, JpegWriter::GRAY);
     else
-      writer = new JpegWriter(JpegWriter::RGB);
+      writer = new JpegWriter(jpegQuality, false, JpegWriter::RGB);
 #endif
   } else if (tiff) {
 #if ENABLE_LIBTIFF