Adding a big file
If your file is too big to upload it to Moodle directly, you may be able to use one of the workarounds below. If you can't use one of these, you'll need to ask me for help.
Keeping PowerPoint files from bloating
The most common case for files getting over 256 MB is PowerPoint files that include video. Unhappily, when you add narration to a PowerPoint, often PowerPoint wants to put a little video of you in the corner to go with your voice. That video—almost always just you as a talking head—is seldom really worth including, and makes the file a lot larger.
Happily, it's not hard to make a PowerPoint with audio only. Microsoft's help gives both the procedure for doing that on an entire presentation, and also the procedure for doing it slide by slide.
Using Google Drive
If your file is still bigger than you can upload to Moodle, you can sometimes upload the file to your Converse Google Drive and share it with the right person or people from there. (You can use link sharing, described on the Google page just linked, to link to the file from Moodle.)