The best format is plain linear HTML (no tables, frames, etc) with any images simply inline; see this example. That flows nicely in any browser. Images should be no wider than 900 pixels.
If HTML is not possible then a PDF is OK but please don't
use the latest Adobe PDF format because many people can't read it. Save the PDF in
some older format e.g. Acrobat 5.
Contact for submissions: firstname.lastname@example.org