is there a tool to decompile PDF file into FO?
Unfortunately this is not possible in a totally generic way. FOP is not a page description language in the sense that it can describe every possible PDF page.
The nearest you can do is to try to extract either the text or the graphics from the PDF and incorporate these in an FO which you develop.
If you want PDF manipulation tools you might want to look at iText (Details in the FAQ or these archives, or google).
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