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I have tested the scenario using following code snippet and as per my observations, the letter 2 is properly being subscripted below CO. The resultant PDF that I have generated with the following code is also in attachment, please take a look.
//Instantiate Pdf object by calling its empty constructor
Pdf pdf1 = new Pdf();
//Create a section in the Pdf object
Aspose.Pdf.Section sec1 = pdf1.Sections.Add();
//Create a new text paragraph with an initial text segment "Aspose"
Text text1 = new Text(sec1,"CO");
//Add the text paragraph to the section
//Create a new text segment into the text paragraph
Segment seg2 = text1.Segments.Add("2");
//Set the vertical alignment of text segment "seg2" to Topline by setting
//IsBaseline property ot seg2.TextInfo to true
// set the font size information for the segment
seg2.TextInfo.FontSize = 5;
//Save the Pdf
I have tested the scenario using Aspose.Pdf for .NET 4.5.0. In case it does not resolve your problem or you have any further query, please feel free to contact. We apologize for your inconvenience.