Double blinded review but the appendix contains author's name Before submitting a review for a manuscript, I noticed the appendix possibly contains an author name, but I don't know him/her. My file explorer automatically shows files' metadata, including author name.

The review should be double blinded. Should I inform the editor?
When in doubt: yes.  Let them know when and how you found this potentially compromising information.  Especially if you think this may impact your objectivity (e.g. "I don't recognize this name, so I'm not going to trust him as much and may/will be more critical than I otherwise would"). But even if you think your evaluation will be unaltered, you should let the editor know.  They need to be the one to decide if the review process can still meet the necessary criteria, if the authors should be notified and asked to alter the file to eliminate the metadata, etc.

In your case it sounds like you did the entire review under appropriate double blinding, fully unaware of the authors while making the evaluation, and then at the very end of the process you gleaned a potential clue about the author name. If so, then the editor may accept that no actual problem has arisen, and may simply try to take steps to ensure such metadata is no longer visible to reviewers.  

Things get more complicated if there are multiple reviewers for this paper: it now becomes possible that one or more of them saw this data early in the process, and it potentially compromised their review, but yet this went unacknowledged. This makes it all the more important to inform the editor.