"When should this be sent out?"
Send the agenda, the meeting material and presentations out to your board members at least four days before the meeting is happening.
"PDF or DOCX"
I would always send out a PDF as it's much easier to open on a mobile phone. Remeber to make all your board documents mobile friendly!
"Do I send it to everyone on the participant list?"
It's good to share the agenda to all the observers also five days before the meeting. Remember to not send sensitive material to observers if they shouldn't be reading the material :)