There should be a "background layer" showing up by default. Shouldn't it?
No. As R.Kelly said, it simply depends on the PDF. If there is no data in there that would have necessitated it to flatten already at the time of creation, thus "baking" the text on the simulated "paper", the PDF content is just "objects" with specific properties that define their appearance. Only things like e.g. using pixel effects such as drop shadows in Illustrator combined with specific PDF output settings when saving may cause the paper simulation to be included as pixel data if there is no other way to retain the result of the transparency blending otherwise. Same for other, similar operations like clipping masks or pixel pattern fills in Word/ Excel documents etc.. That is to say what you see is normal - documents just containing a bunch of text will be created by embedding vector data and text objects to eep their size small and PS will rasterize them this way as well...
Mylenium