You can use the Yahoo Backup Tool
is an outstanding utility that is well capable of converting Yahoo emails into PDF, PST, EML, EMLX, MBOX, MSG, HTML, MHTML, JSON, etc. It migrates Yahoo mailboxes into Gmail, Office365, Thunderbird, Zoho Mail, GoDaddy, Yandex Mail, AOL, etc. They allow the users to free up the server space of Yahoo Mail and users can skip their previously migrated items from the conversion process. Addtionally, it is compataible with all versions of the windows operating systems. You can download the free demo version of the software to analyze the performance.
However, if you want to transfer Yahoo mail to Office 365 then you will go for the informative article Migrate Yahoo Mail To Office 365
. Here you will get the most effective manual method to complete the migration process.