We have tested PDF Stamping with files as large as 100 MB, so it should work with an 80 MB file, though it may take a few minutes after uploading the file before it can be stamped.
You might also consider whether "optimizing" your PDF might reduce the file size -- e.g., resampling high-resolution images to suit the size they're displayed at in the PDF can result in a significant filesize reduction with no evident loss of quality. Your PDF authoring program may have settings to do this automatically.
