PDFBox is the GOAT of backend pdf libraries. We've built incredible things with it, plus pdfjs on the front-end - full compliant e-signature, templated pdf generation, in-browser pdf editing. Looked deeply at alternatives but very happy with our choice. In particular using itext vs pdfbox feels like using WordPress vs Rails - try to build anything very serious and you will be happier you picked the more capable, lower-level library.