Put all the messages from a Gmail inbox satisfying a query into a Google document

collate_emails.js

function collateMessages() {
  var searchString = "YOURQUERYHERE";
  // A query can be anything that you can enter
  // into the Gmail search field:
  // "datageddon from:ivan@pochta.net filename:pdf"
  // etc.
  var doc = DocumentApp.create(searchString);
  var threads = GmailApp.search(searchString);
  for (var j = 0; j < threads.length; j++) {
    var messages = threads[j].getMessages();
    for (var i = 0; i < messages.length; i++) {
      var txt = messages[i].getPlainBody();
      doc.getBody().appendParagraph(txt);
      doc.getBody().appendParagraph("");
    }
  }
}