I'm a PhD student and I prepared a poster for a conference. Do you think I should send the poster file to all the co-authors? The list is quite long so I mailed the draft only to a few of them.
Yes. Everybody who is listed as an author should see and be able to contribute to the poster. You wouldn't want to be listed as an author of a work that you have never seen either, right?
Completely agree with xLeitix's answer:
Everybody who is listed as an author should see and be able to contribute to the poster.
Also, if it is a big file (assuming your poster is heavy on graphics) perhaps it is best to upload it to dropbox or a similar service and then send the link to all the authors. Depending on their (and your) digital fluency, the team can refine the document right from dropbox without having to send email back and forth.