You are getting sent to Dropbox because the size of the attachments exceeds Yahoo’s limits. If you don’t like Dropbox (not sure why ... it is a REALLY good service), pick your own Cloud Storage Solution. Microsoft has OneDrive that is free to anyone with an O365 Account, Apple has one that I think is free if you own an Apple product. Basically, store the doc in the cloud, then share the directory with your friend. They simply get a link, and download at their leisure, and the email servers don’t get dropped to their knees.
Welcome to the future ... and if you don’t conform ... twas nice knowing ya ... companies are getting smarter about deploying infrastructure and taking on costs, and just cause they used to let you send massive emails doesn’t mean they ever will again, especially if you are using a free service.