I'd imagine rendering as HTML is the safest bet. PDF rendering definitely depends on the target machine. The new Windows Mobile 5.0 devices usually come with a PDF viewer (my Cingular 8125 has "ClearVue PDF"), but I haven't tested their ability to view a Workflow-rendered XDP as of yet... I'll put that on my list for this weekend and get back to you. A tablet PC shouldn't have any problems, as it's probably running standard Windows apps and can do anything in-browser that a regular PC could do.