this is a sensational finding, that no-one in the world knows! :)
i searched for this bug crawling very deep, but in the entire internet everyone has the problem and no-one grasps the right answer.

when saving a .pdf file from a .psd file in adobe photoshop, check your text layers!

if they contain faux photoshop generated text properties (like faux bold, faux italic and in particular faux underline) probably you will get raster text in your .pdf file, instead of vector text (ok, i know: it's postscript).
this means: instead of getting the kind of text you can easily select, highlight, copy and paste where you need, photoshop outputs a .pdf displaying text that can't be selected (you know, when mouse cursor does not change when hovering etc...). this is actually a bitmap image, technically speaking a raster version of the text.

that's it: five hours of time wasted in wondering wtf*! went wrong, in copying and pasting in notepad to delete text formatting, in reading about compatibility issues from cs2, cs3, cs5... and i was missing a single faux-underlined letter.

ps: very few people notice and understand this problem when generating a .pdf binder. locked or raster text renders unuseful the entire concept of writing an e-book, as users and search engines cannot scan its content.
this fact rules out how much we care about our work here at amolenuvolette.it: thank you in advance if you will share your appreciation.