Can PDFNet split PDF, add bookmars, new content?

Q: I'd like to know if you can offer me some informations about one of
your products, pdf SDK. I'd like to know if the SDK supports the
following operations on PDF documents:

- join more PDF files in one new file;
- create new bookmarks based on some custom rules;
- create internal and external links;
- add graphic elements inside a PDF document (draw circles, rectangles,
lines); all this, independent of any Adobe SDK.
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A:
Yes, you can use PDFNet SDK (http://www.pdftron.com/net) to implement
all of your requirements.

join more PDF files in one new file;

Yes, using PDFNet you can also split and merge multiple PDF
documents. For more information, please see the following resources:

PDFPage sample project:

Page manipulation overview:

create new bookmarks based on some custom rules;

Please see Bookmark sample project:

create internal and external links;

A good starting point for this would be Annotation sample:
  http://www.pdftron.com/net/samplecode.html#Annotation

add graphic elements inside a PDF document (draw circles,
rectangles, lines); all this, independent of any Adobe SDK.

As a starting point you may want to check the following links:
'How do I stamp a page?':

Also ElementBuilder sample project: