This can become difficult if you only need to place this signature on one page of a multiple page document. If it is only one page or if the signature needs to be place on every page you can create a custom pdf with the image on the right location. Then use the Community commons OverlayPdfDocument to combine the page(s). Otherwise you would need custom Java code to cut the PDF up in pages and combine them back again. But that is no easy feat.
If you need more advanced placement of the signature image, especially when dealing with multiple pages, you might need to use custom Java code.
Manipulating PDF pages programmatically can be complex, so this approach may require a good understanding of PDF internals and the iText library or similar PDF manipulation libraries in Java.
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