context.Credentials = new SharePointOnlineCredentials("xxx@xyz.onmicrosoft.com", password);
Web web = context.Web;
context.Load(web);
context.ExecuteQuery();
List docList = web.Lists.GetByTitle("WikiDocuments");
context.Load(docList);
CamlQuery camlQuery = new CamlQuery();
camlQuery.ViewXml = "<View Scope='RecursiveAll'></View>";
ListItemCollection listCol = docList.GetItems(camlQuery);
ListItemCollection items = listCol;
context.Load(items, icol => icol.Include(i => i["PublishingPageContent"]));
context.ExecuteQuery();
string html;
foreach (ListItem item in items)
{
    File file = item.File;
    var wikiItem = file.ListItemAllFields;
    context.Load(wikiItem);
    context.ExecuteQuery();
    html = wikiItem["PublishingPageContent"].ToString();
}