hus, free applications do not normally implement “features” which allow their users to do less. One might think that the consensus against “antifeatures” in free software is nearly universal, but, as the case of the okular PDF reader in Debian shows, there are still exceptions. More here
This is not the case with Okular, though. An attempt to select text from a suitably-marked PDF file yields a rather confusing dialog which reads “copy forbidden by DRM” (see the image to the right). Amusingly, the application will still allow the selected region to be saved as an image file, but sending the text to the clipboard is not allowed.