For PDFs it's hard to say, as it's internally not necessarily text,
but can be quite graphical (depending on the program that created the
PDF in the first place). PDF, in its core, is Postscript - so there
might be all kinds of escape sequences and positioning information in
between single letters/glyphs.
Try to extract text with some of the available tools or services and
see what it reveals. It might show that your PDF indeed is not text,
but an image.
