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A lot of PDF's aren't text but are images, pictures of text. You can convert them but they can't do many of the things books of text can do, such as flow. I suspect that would also hamper them as dictionaries.
Not all PDF's are images so this might not be your problem but most are images. It's worth checking out.
A quick (but not always valid) check to see if a PDF is images is to check the file size. They're usually many times the size a comparable length book would be if it was text. Of course books with text can also contain images so this can be misleading. But it's usually a good first check.
Barry
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