epub to pdf: unable to select text after conversion

I'm converting an epub file to a pdf file using ebook-convert. The output is fine but after opening it with acrobat reader I'm unable to select text, it doesn't even recognize word limits. It's as if the underlying word structure was lost in translation and now the reader just sees a stream-of-characters. Why is this so? The input epub has a notion of stream-of-words, why was it lost during conversion?

Thank you
--Carlos