OK, so I was creating a PDF with Inkscape using images from the web. I copied them and pasted them all in Inkscape, nothing strange, but then every time I copied a new image to the Inkscape workspace somehow the images that were already there got replaced with the image I just copied. I don't know if it's a feature of Inkscape or something like that, or could it be Ubuntu.
Well, I certainly don't know what it is but it's rather annoying.
Also this affects the PDF files that were already created. Say I have a PDF with an image of a dog, and I click copy and choose the image of a cat, the PDF file gets edited, even when it's not in use.