[Gimp-user] GIMP, .pdf, and threshold

[m roth 5-cent us]

Hi, folks,

   I'm running CentOS 6.7, and the most current GIMP for that, 2.6.9-8. A
friend, half a continent away, is scanning parts of a number of old
'zines for me. From that, The originals were printed on colored
paper....

    I want to do OCR. On a couple of .pdfs, one I d/l from the 'Net, and
one from a different scanner, I import it into GIMP as images (we're
talking 16-20pp), and, page by page, push the contrast to max, cut the
brightness to half or less of what it was, then pull up threshold, and
the preview shows me black text on white, and saving it that way,
tesseract works just fine.

   Doing that with these scans, nada. No useful changes, and threshold
does nothing.

   Clues for the poor?

        mark