PM> Install-Package Spire.Doc

using Spire.Doc;
using Spire.Doc.Documents;
using Spire.Doc.Fields;
using System.Drawing;

namespace AddFootnote
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a Document instance
            Document document = new Document();

            //Load a sample Word document
            document.LoadFromFile("Sample.docx");

            //Get the first section
            Section section = document.Sections[0];

            //Get a specified paragraph in the section
            Paragraph paragraph = section.Paragraphs[3];

            //Add a footnote at the end of the paragraph
            Footnote footnote = paragraph.AppendFootnote(FootnoteType.Footnote);

            //Set the text content of the footnote
            TextRange text = footnote.TextBody.AddParagraph().AppendText("Algorithms can be simple or complex depending on what you want to achieve.");
            
            //Set the text font and color
            text.CharacterFormat.FontName = "Arial";
            text.CharacterFormat.FontSize = 12;
            text.CharacterFormat.TextColor = Color.DarkBlue;

            //Set the format of the footnote superscript number
            footnote.MarkerCharacterFormat.FontName = "Calibri";
            footnote.MarkerCharacterFormat.FontSize = 15;
            footnote.MarkerCharacterFormat.Bold = true;
            footnote.MarkerCharacterFormat.TextColor = Color.DarkCyan;

            //Save the result document
            document.SaveToFile("AddFootnote.docx", FileFormat.Docx);

        }
    }
}

Imports Spire.Doc
Imports Spire.Doc.Documents
Imports Spire.Doc.Fields
Imports System.Drawing

Namespace AddFootnote
    Class Program
        Private Shared Sub Main(ByVal args() As String)
            'Create a Document instance
            Dim document As Document = New Document

            'Load a sample Word document
            document.LoadFromFile("Sample.docx")

            'Get the first section
            Dim section As Section = document.Sections(0)

            'Get a specified paragraph in the section
            Dim paragraph As Paragraph = section.Paragraphs(3)

            'Add a footnote at the end of the paragraph
            Dim footnote As Footnote = paragraph.AppendFootnote(FootnoteType.Footnote)

            'Set the text content of the footnote
            Dim text As TextRange = footnote.TextBody.AddParagraph.AppendText("Algorithms can be simple or complex depending on what you want to achieve.")

            'Set the text font and color
            text.CharacterFormat.FontName = "Arial"
            text.CharacterFormat.FontSize = 12
            text.CharacterFormat.TextColor = Color.DarkBlue

            'Set the format of the footnote superscript number
            footnote.MarkerCharacterFormat.FontName = "Calibri"
            footnote.MarkerCharacterFormat.FontSize = 15
            footnote.MarkerCharacterFormat.Bold = True
            footnote.MarkerCharacterFormat.TextColor = Color.DarkCyan

            'Save the result document
            document.SaveToFile("AddFootnote.docx", FileFormat.Docx)
        End Sub
    End Class
End Namespace

using Spire.Doc;
using Spire.Doc.Documents;
using Spire.Doc.Fields;
using System.Drawing;

namespace InsertFootnote
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a Document instance
            Document document = new Document();

            //Load a sample Word document
            document.LoadFromFile("Sample.docx");

            //Find a specified text string
            TextSelection selection = document.FindString("big O notation", false, true);

            //Get the text range of the specified text
            TextRange textRange = selection.GetAsOneRange();

            //Get the paragraph where the text range is located
            Paragraph paragraph = textRange.OwnerParagraph;

            //Get the position index of the text range in the paragraph
            int index = paragraph.ChildObjects.IndexOf(textRange);

            //Add a footnote
            Footnote footnote = paragraph.AppendFootnote(FootnoteType.Footnote);

            //Insert the footnote after the specified paragraph
            paragraph.ChildObjects.Insert(index + 1, footnote);

            //Set the text content of the footnote
            TextRange text = footnote.TextBody.AddParagraph().AppendText("It gives the worst-case complexity of an algorithm.");

            //Set the text font and color
            text.CharacterFormat.FontName = "Arial";
            text.CharacterFormat.FontSize = 12;
            text.CharacterFormat.TextColor = Color.DarkBlue;

            //Set the format of the footnote superscript number
            footnote.MarkerCharacterFormat.FontName = "Calibri";
            footnote.MarkerCharacterFormat.FontSize = 15;
            footnote.MarkerCharacterFormat.Bold = true;
            footnote.MarkerCharacterFormat.TextColor = Color.DarkGreen;

            //Save the result document
            document.SaveToFile("InsertFootnote.docx", FileFormat.Docx);
        }
    }
}

Imports Spire.Doc
Imports Spire.Doc.Documents
Imports Spire.Doc.Fields
Imports System.Drawing

Namespace InsertFootnote
    Class Program
        Private Shared Sub Main(ByVal args() As String)
            'Create a Document instance
            Dim document As Document = New Document

            'Load a sample Word document
            document.LoadFromFile("Sample.docx")

            'Find a specified text string
            Dim selection As TextSelection = document.FindString("big O notation", False, True)

            'Get the text range of the specified text
            Dim textRange As TextRange = selection.GetAsOneRange

            'Get the paragraph where the text range is located
            Dim paragraph As Paragraph = textRange.OwnerParagraph

            'Get the position index of the text range in the paragraph
            Dim index As Integer = paragraph.ChildObjects.IndexOf(textRange)

            'Add a footnote
            Dim footnote As Footnote = paragraph.AppendFootnote(FootnoteType.Footnote)

            'Insert the footnote after the specified paragraph
            paragraph.ChildObjects.Insert((index + 1), footnote)

            'Set the text content of the footnote
            Dim text As TextRange = footnote.TextBody.AddParagraph.AppendText("It gives the worst-case complexity of an algorithm.")

            'Set the text font and color
            text.CharacterFormat.FontName = "Arial"
            text.CharacterFormat.FontSize = 12
            text.CharacterFormat.TextColor = Color.DarkBlue

            'Set the format of the footnote superscript number
            footnote.MarkerCharacterFormat.FontName = "Calibri"
            footnote.MarkerCharacterFormat.FontSize = 15
            footnote.MarkerCharacterFormat.Bold = True
            footnote.MarkerCharacterFormat.TextColor = Color.DarkGreen

            'Save the result document
            document.SaveToFile("InsertFootnote.docx", FileFormat.Docx)
        End Sub
    End Class
End Namespace

using Spire.Doc;
using Spire.Doc.Documents;
using Spire.Doc.Fields;

namespace RemoveFootnote
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a Document instance
            Document document = new Document();

            //Load a sample Word document
            document.LoadFromFile("AddFootnote.docx");

            //Get the first section
            Section section = document.Sections[0];

            //Traverse through each paragraph in the section to find the footnote
            foreach (Paragraph para in section.Paragraphs)
            {
                int index = -1;
                for (int i = 0, cnt = para.ChildObjects.Count; i < cnt; i++)
                {
                    ParagraphBase pBase = para.ChildObjects[i] as ParagraphBase;
                    if (pBase is Footnote)
                    {
                        index = i;
                        break;
                    }
                }

                if (index > -1)

                    //Remove the footnote
                    para.ChildObjects.RemoveAt(index);
            }

            //Save the result document
            document.SaveToFile("RemoveFootnote.docx", FileFormat.Docx);
        }
    }
}

Imports Spire.Doc
Imports Spire.Doc.Documents
Imports Spire.Doc.Fields

Namespace RemoveFootnote
    Class Program
        Private Shared Sub Main(ByVal args() As String)
            'Create a Document instance
            Dim document As Document = New Document

            'Load a sample Word document
            document.LoadFromFile("AddFootnote.docx")

            'Get the first section
            Dim section As Section = document.Sections(0)

            'Traverse through each paragraph in the section to find the footnote
            For Each para As Paragraph In section.Paragraphs
                Dim index As Integer = -1
                Dim cnt As Integer = para.ChildObjects.Count
                Do While (i < cnt)
                    Dim pBase As ParagraphBase = CType(para.ChildObjects(i), ParagraphBase)
                    Dim i As Integer = 0
                    If (TypeOf pBase Is Footnote) Then
                        index = i
                        Exit For
                    End If

                    i = (i + 1)
                Loop

                If (index > -1) Then
                    para.ChildObjects.RemoveAt(index)
                End If

            Next

            'Save the result document
            document.SaveToFile("RemoveFootnote.docx", FileFormat.Docx)
        End Sub
    End Class
End Namespace