If you need to align at the text level you'll need to switch to a fixed-width font. But if you're just looking to indent you can just add spaces to new lines within a paragraph:
var p = new Paragraph();
p.Add("First line text\n");
p.Add(" Second line text\n");
p.Add(" Third line text\n");
p.Add("Fourth line text\n");
myTable.AddCell(p);
You could also get complicated and use a sub-table if you need more control:
var subTable = new PdfPTable(new float[] { 10, 100 });
subTable.AddCell(new PdfPCell(new Phrase("First line text")) { Colspan = 2, Border = 0 });
subTable.AddCell(new PdfPCell() { Border = 0 });
subTable.AddCell(new PdfPCell(new Phrase("Second line text")) { Border = 0 });
subTable.AddCell(new PdfPCell() { Border = 0 });
subTable.AddCell(new PdfPCell(new Phrase("Third line text")) { Border = 0 });
subTable.AddCell(new PdfPCell(new Phrase("Fourth line text")) { Colspan = 2, Border = 0 });
myTable.AddCell(subTable);