Hi @Max358,
You can also attach the review files to the email notification to the author, always making sure they fit the review format the journal works with (open, blind, double-blind), or you could upload a new file and attach it to the email.
An alternative to this is opening a discussion and sending it to the author with a message body and attached files, this will stay on the submission’s page, it won’t be visible to reviewers, and also send an email notification to the author.
I hope this information is useful.
Best
Alejandra