As you are seeing unwanted symbols in your PDF file instead of valid text data, this means file has some corruption issues which can not be fixed manually. You need an advanced recovery tool as this would be the last option you are left with.
I can recommend you to try demo version of SysInfoTools PDF Repair
tool to preview the recovered data before actual recovery. If you can see your valid text data in the preview window, you can buy its full version to save the data.
To know more about this software and to download its free demo version, you may visit: www.sysinfotools.com/pdf-repair