If there are limits to what PDFs work and what don't I have not run into them :)
You've just been lucky so far :)
Some PS/PDF tools are using font subsetting/re-encoding techniques
which (when done in a certain way) can make automatic text extraction
very hard. (I've tried to explain the method in more detail in
another thread.)
To illustrate, here's a sample PDF
which you can view with Adobe Reader, xpdf, Ghostscript, etc. without
problems (you should see the standard "lorem ipsum" text). Any attempt
to extract the textual content will likely fail, however, although the
file is a perfectly valid PDF containing nothing but regular text
content (no images, no encryption, no other tricks) with all characters
being part of the ASCII set.
Of course, I deliberately created the file in the above mentioned
way (as "proof of concept"), but there are actually PDF creation tools
out there which do produce such problematic PDFs.
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