In this release, we've added functionality that allows you to completely remove an optional content layer (OCG). However: iText 5.1.2 is mainly a maintenance release that fixes a series of bugs reported on this list since June 8: the algorithm to find startxref was made more memory efficient; the height of the Image wasn't taken into account when creating a PdfPCell with an Image (this is a bug that was introduced in iText 5.1.1); fixed a problem when an XFA stream didn't contain a datasets node; fixed a bug in the signature validation process; fixed a bug where iText introduced the namespace prefix "xmp" when "xap" was expected; and so on.
This release also allows us to release a new version of XML Worker:
XML Worker 1.1.0
In this release we've added support for more HTML tags and CSS attributes, but the main difference is a change in the architecture. If you were using previous versions of XML Worker, you'll need to make some changes to your own code after upgrading to XML Worker 1.1.0. The change in the architecture was necessary to fix some threading problems. Working with byte instead of String also turned out not to be a good idea.
the iText team