Your PDF only contains "scanned images" and not vector objects (like lines, arcs, text, etc.). ACE can only convert true PDF & Postscript files that contain vector objects, and not embedded images.
Please checkout our recommended PDF Conversion Guidelines:
Note: We would recommend that you use the free graphics tool The Gimp (or Adobe Illustrator), to open that PDF and save it as a bitmap (make sure to use 1200 dpi or higher while saving). Then use the ACE "Large Bitmap" importer to load the bitmap file and convert to gerber or whatever format you require.
Numerical Support Team
Note: We would recommend that you use the free graphics tool The Gimp (or Adobe Illustrator), to open that PDF and save it as a bitmap (make sure to use 1200 dpi or higher while saving). Then use the ACE "Large Bitmap" importer to load the bitmap file and convert to gerber or whatever format you require.