Calibre should be able to convert it to epub without any issues quite easily. What do you mean by *not even openable*. Do you see the file on the eink screen of the nook, and when you open it any errors to report ?
Open your pdf and try to use medium font size normally it should reflow it.
If your pdf is scanned images there isn't nothing you can do about it, just wait and purchase a soon to come 10 inch reader (QUE or Skiff).
Happy reading.
planetrobbie
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