Answer:
Probably "compress", but these days the common answer is "upload to cloud".
Explanation:
Compressing the files is an easy way to reduce their size, unless most of the size is in already compressed, high-entropy formats (like mp3, jpeg or mp4).
The common compression format is .ZIP - you've probably seen it countless times, but other ways like RAR, 7Z are also popular, while Linux users mostly deal with tar.gz, tar.bz2 or tar.xz
On the other hand, the standard practice these days is to upload the presentation to a cloud service, like GSheets or Office PowerPoint 365, which gets rid of the limits of email filesize, while providing a convenient web-app way to view the presentation without downloading (and it doesn't clutter their inbox space or hard drives)! Alternatively, one other way to email any large file (not just a presentation) includes uploading it to some service like DropBox, GDrive or anything similar.
