Preview (copy text) results in SQUIGGLES not text for certain PDFs

hi - problem is "copy text and paste from certain PDFs result in squiggles

When i use an online converter service (e.g. http://ebook.online-convert.com/convert-to-pdf) to convert files to PDF, Apple Preview does not work properly.

What happens is, when I view this PDF and highlight the text to copy in Apple Preview (snowleapard 10.6.x), and then paste the text say in an email / text file or word doc ---- i get squiggles NOT text.

However, if I view this same PDF in Adobe Pro, I get text (copy text and paste works)

I can also view this same PDF in my iphone app, Goodreader, and "copy text and paste) to email works e.g. I get text.

Interestingly, if I create the PDF NOT from this online service, but say from Adobe PRO, Apple PReview works (e.g. I copy and paste the text no issue)

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is this a known bug of Apple Preview or is there some setting i must change somewhere?

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the support/developer dude of the online service keeps telling me its an Apple Preview problem, not a problem of his service.....

thankyou in advance for your help.