Is iText 5.4.2 open source?

AGPL stands for Affero GPL.

It basically says, when you want to sell a software product that includes iText or use iText in any commercial webservice, than you need to buy a license.

If you do an open source (with an approriate license) or private project, than you are fine and don't need to buy a license.

iText 5.4.2 is open source, because the source is openly available for everybody - so the answer to your question is yes. However your question and your definition of open source might be a different one:

Depending on the license you are able to use an open source project in a commercial project without the need to also opensource the whole source code of that project under that license.

If that was the question then no: AGPL licensed projects can not be used in commercial projects (without open sourcing the commercial project which is a nogo for most commercial projects)

The last iText version released under a less restrictive license (commercially usable) was iText 2.1.7 (java) and 4.1.6 (c#). It was both released under MPL / LGPL. Even though the authors removed it from the sourceforge page you are able to find it in many places in the web (e.g. maven central). Those versions are from 2009 and still do their job fine, if you need the latest pdf features however (newer versions of digital signatures, ...) or experience a bug you may have to use the latest version...

Update: There was an unofficial java version 4.2.0 but you have to build it for yourself.

Update II: There is a fork called 'OpenPDF' which continues based on that version. Today I would use that project.