You have added the text object into the section and then added it into cell again which is not allowed. Please look at the following code:
Dim myPDF As Pdf = New Pdf
myPDF.Sections.Add()
Dim section2 As Section = myPDF.Sections(0)
Dim text2 As Text = New Text(section2)
'section2.Paragraphs.Add(text2)
Dim segment2 As Segment = text2.Segments.Add("this is a web link")
segment2.Hyperlink = New Hyperlink
segment2.Hyperlink.Url = "http://localhost/popup.htm"
segment2.Hyperlink.LinkType = HyperlinkType.Web
Dim summaryTable As Table = New Table
section2.Paragraphs.Add(summaryTable)
summaryTable.ColumnWidths = "120"
summaryTable.DefaultCellBorder = New BorderInfo(BorderSide.All)
Dim row1SummaryTable As Row = summaryTable.Rows.Add
Dim cell3Row2SummaryTable As Cell = row1SummaryTable.Cells.Add
cell3Row2SummaryTable.Paragraphs.Add(text2)
myPDF.Save("d:/test/test.pdf")