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Is it possible to overlay a page from one PDF onto another page in different PDF?

This is a quick and relatively simple way to overlay one page onto another. This is sometimes called stitching, overlaying, watermarking, etc, -- although each of those terms can refer to slightly different tasks as well.

The benefit of overlaying one page onto another the way I've outlined below is that you don't have to render either of the pages as an image, so in the final output, you'll still be able to select individual elements on the page, including text.

I've used Delphi for this sample, but the syntax is quite straightforward and it shouldn't be difficult to understand.

procedure TForm2.btnOverlayClick(Sender: TObject);
var
  FileA: Integer;
  FileB: Integer;
  CapturedPageID: Integer;
  PageHeight: Double;
  PageWidth: Double;
  Path: AnsiString;
begin
  QP := TQuickPDF0716.Create;
  try
    UnlockResult := QP.UnlockKey('...'); // Insert trial or paid license key here
    if UnlockResult = 1 then
    begin
    // You can only use the DrawCapturedPage function if the captured page is in the same
    // document, so first we'll need to merge the two pages that we wish to overlay together
    // into one document.
    if dlgOpen.Execute then
      begin
         QP.LoadFromFile(dlgOpen.FileName);
         FileA := QP.SelectedDocument();
      end;
      if dlgOpen.Execute then
      begin
         QP.LoadFromFile(dlgOpen.FileName);
         FileB := QP.SelectedDocument();
      end;
      // After merging FileB is automatically deleted, leaving only FileB which is now a combination of FileA and FileB
      QP.SelectDocument(FileA);
      QP.MergeDocument(FileB);

      // Capture the second page in the merged document
      CapturedPageID := QP.CapturePage(2);
      // Now select the first page and retrieve its height and width
      QP.SelectDocument(FileA);
      QP.SelectPage(1);
      PageHeight := QP.PageHeight();
      PageWidth := QP.PageWidth();
      // Draw the captured page onto the currently selected page
      QP.DrawCapturedPage(CapturedPageID, 0, PageHeight, PageWidth, PageHeight);

      // Save the stitched file to disk
    if dlgSave.Execute then
      begin
        Path := dlgSave.FileName;
      end;
      QP.SaveToFile(Path);
    end;
  finally
    QP.Free;
  end;
end;

Obviously if the two pages don't have the same dimensions then you'll need to do some tinkering, but if they do, then this code should work quite well.