[Comfy.n]

FWIW, I did some rounds of tests with the suggested pdfinfo.exe replacement, to see if I would get more ISBN detections than with the built-in one. Conclusion was: I get the same results, using either.

On this specific round, I've used the save to single folder Calibre feature and ran a PDF-Xchange search on all PDFs in that directory for "ISBN", and here's what I found (I've pasted the search results):

ISBN 978-85*275-0036-4
ISBN 0S-2CM - 1655-1
ISBN 978-I-'77046-048-5
ISBN 978 -S 5 -204-43 14-9

Those were mere OCR errors, there's a wildcard, there's an I instead of 1, some random apostrophe and so on.

ISBN 1-56924-647-5
ISBN: 0-595-26912-5
ISBN 0-7710-8157-X
ISBN 0-664-25513-2
ISBN 0-07-056317-9
ISBN Number 0-913878-50-2
ISBN 85-7326·196-X

The above seem to be valid ISBN-10, though I'm not really familiar with ISBN specs, can't really tell what could have impeded correct detection

ISBN-13: 978-1-5095-4619-0
ISBN-13: 978-1-5095-4620-6
ISBN 978—0—8254—2666-7
ISBN 978-85-401-5414-1
ISBN 9780099511564
ISBN 978-85-7622-857-8
ISBN 978-85-204-4224-1

And these were the undetected ISBN-13.

Out of 27 PDFs containing ISBNs, the plugin has detected correctly the ISBN in 9 of them, so I wouldn't say "it doesn't work with PDFs"!