- Convert the PDF document to an image with File » Save As.... Choose PNG as the format and change the resolution as desired.
- Open the image file and select the whole image with Edit » Select All (or select just a part of it with the mouse if you wish).
- Copy the selection Edit » Copy.
- Go back to the document (which is now a PNG image) and paste the selection with Edit » Paste and resize it as you wish.
- Save the file as a PDF with File » Save As.... Choose PDF as the format (and rename it if the original PDF needs to be preserved).
[crarko adds: I tested this, and it works as described. The original article included with the submission is here.]

