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If you could do this to pdf, I might be willing to pay little for it.

pdf is not reflowable. So I don't think it's possible while still adhering to the pdf standard.

A PDF extractor could be used first, to get the content to be in the same standard as required, and then your tool could make it scroll this way.

This would be quite useful to me as a PDF reader.

Actually it seems perfectly doable with a custom pdf reader.