This reduces the size of one or more PDF files. Do this using Home > Tools > Reduce then choose Reduce current file or Reduce multiple files. Compile a file list, click OK then choose a target folder (same as original or a different one) and a file name solution (keep original file names or add a prefix or suffix to the original file names). Choose whether or not to overwrite existing files. Reduction offers no choice on what to remove.
Reduction cannot cause data loss, and does not compress images. Comments are not removed by file reduction. Reduction works by removing unseen but duplicated information from the file – the level of reduction is strongly file-dependent.
File optimization can be chosen from the dropdown menu. It also aims to reduce file size, but does this mainly by compressing images (to levels you can set) and removing certain types of file content that you can choose.