Yep, and in that case it's because those PDFs were often generated through really horrifying pipelines (e.g. TeX to DVI to PS to PDF). Under some workflows, the

Yep, and in that case it's because those PDFs were often generated through really horrifying pipelines (e.g. TeX to DVI to PS to PDF). Under some workflows, the resulting document wouldn't even contain any characters, as far as PDF was concerned -- it'd just be a bunch of vectors.