Here's what I'm experiencing...
Let's say I create a poster with a couple of blocks of text in Illustrator, using a single font. Then, I generate a PDF of the poster.
Now, on the same computer, I open the PDF in Illustrator, and Illustrator pops-up an alert saying some text has been outlined to preserve appearance. And, indeed, here and there throughout the text, some words, combination of words, or whole lines of text have been outlined, but most of the text is still live.
Why does this happen? There's no difference, font-wise, between the text that is untouched, and the text that got outlined. Same face, same weight. In fact, the same word, set the same way, will be untouched in one line, and be outlined elsewhere in the text. It's really baffling.
