The current version of the review forum does not support file exchange between reviewers and authors. If you would like to send a file with your comments to the authors we would advise to either contact the handling editor by clicking on the little envelop or email the editorial office. When doing so, please ensure you only send anonymized PDFs/Docs and please also ensure you CC the Editorial Office.
However, why not make the most of the review forum? The review forum is designed so that it facilitates the interaction, and direct exchange of ideas between reviewers and authors. The dedicated questionnaire serves as a guide for the review and helps focus on the scientific accuracy of the manuscript rather than its potential impact. Further, the review forum makes bookkeeping of different versions of the manuscript and comments easier, since everything is concentrated in one page. See, how you can access the manuscript and submit your report online.