Suppose that you have a
PdfAnnotation object in iText 7. To lock the annotation defined by this annotation dictionary, you need to set the
PdfAnnotation.LOCKED flag, for instance with the
Note that using this method will override the flags that were already defined before.
As for the opacity, that's define by the
ca entry of the annotation dictionary.
annot.put(PdfName.ca, new PdfNumber(0.27));
My snippets are written in Java. You'll need to apply small changes to the methods if you want to use them in C# code.
Click How to change the properties of an annotation? if you want to see how to answer this question in iText 5.