In Adobe Premier Pro, there is a dropdown list of formats to choose from. I normally use H264 for Youtube. I save the exported video to my hard drive first, then upload it later to Youtube, Vimeo.

Here are the guidelines for what Vimeo wants. https://vimeo.com/help/compression

These are the detailed settings for youTube. https://support.google.com/youtube/answer/1722171 I use the same settings for youTube as I use for Vimeo.

For dropbox I'd use the same quality as for Youtube, but the format you provide will depend on what most people downloading the video commonly use.

A video forum may be able to give you more detailed help.