iText is for creating, consuming and manipulating PDF files and iTextSharp is the .NET library for iText 5. These PowerShell scripts need iTextSharp to run. So first things first: Get iTextSharp e.g. from Nuget: https://www.nuget.org/packages/iTextSharp/

Rename the file from .nupkg to .zip, unpack the itextsharp.dll and copy it to the path of the script.

The script in iText7 shows how it works to create a PDF file with PowerShell and the new iText 7, because iText 5 or iTextSharp is EOL, and has been replaced. Only security fixes will be added. It is highly recommended to use iText 7 for new projects, and to consider moving existing projects from iTextSharp to iText 7.