Hi Giampaolo, we've looked at the samples you sent. When printing to dopdf directly from the web, text is sent to it as text, and images as images. However, when printing another pdf to dopdf, text is sent as images too and they overlap for different text sections, that's why that problem occurs.
We'll try to correct this in the future but since this is something that very few people requested it's not going to be a priority - as I told you the idea is that it doesn't make that much sense to print a pdf to another pdf. Thing is that even in your case, you can create the pdf file from the web and after that use the free Adobe Reader to add sticky notes to it.
As for the other 2 programs you mentioned, they both use Ghostscript so that's why for them it's the same result, because it's not something they fixed or developed is something that ghostscript offers.