PM> Install-Package Spire.Doc

using System;
using Spire.Doc;
using Spire.Doc.Documents;

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

            //Load a sample Word document
            original.LoadFromFile(@"E:\Files\SplitByPageBreak.docx");

            //Create a new Word document and add a section to it
            Document newWord = new Document();
            Section section = newWord.AddSection();
            int index = 0;

            //Traverse through all sections of the original document
            foreach (Section sec in original.Sections)
            {
                //Traverse through all body child objects of each section
                foreach (DocumentObject obj in sec.Body.ChildObjects)
                {
                    if (obj is Paragraph)
                    {
                        Paragraph para = obj as Paragraph;
                        sec.CloneSectionPropertiesTo(section);

                        //Add paragraph object in the section of original document into section of new document
                        section.Body.ChildObjects.Add(para.Clone());

                        //Traverse through all child objects of each paragraph and determine whether the object is a page break
                        foreach (DocumentObject parobj in para.ChildObjects)
                        {
                            if (parobj is Break && (parobj as Break).BreakType == BreakType.PageBreak)
                            {
                                //Get the index of page break in paragraph
                                int i = para.ChildObjects.IndexOf(parobj);

                                //Remove the page break from its paragraph
                                section.Body.LastParagraph.ChildObjects.RemoveAt(i);

                                //Save the new Word document
                                newWord.SaveToFile(String.Format("result\out-{0}.docx", index), FileFormat.Docx);
                                index++;

                                //Create a new document and add a section
                                newWord = new Document();
                                section = newWord.AddSection();

                                //Add paragraph object in original section into section of new document
                                section.Body.ChildObjects.Add(para.Clone());
                                if (section.Paragraphs[0].ChildObjects.Count == 0)
                                {
                                    //Remove the first blank paragraph
                                    section.Body.ChildObjects.RemoveAt(0);
                                }
                                else
                                {
                                    //Remove the child objects before the page break
                                    while (i >= 0)
                                    {
                                        section.Paragraphs[0].ChildObjects.RemoveAt(i);
                                        i--;
                                    }
                                }
                            }
                        }
                    }
                    if (obj is Table)
                    {
                        //Add table object in original section into section of new document
                        section.Body.ChildObjects.Add(obj.Clone());
                    }
                }
            }

            //Save to file
            newWord.SaveToFile(String.Format("result/out-{0}.docx", index), FileFormat.Docx);

        }
    }
}

Imports Spire.Doc
Imports Spire.Doc.Documents

Namespace SplitByPageBreak
    Class Program
        Private Shared Sub Main(ByVal args() As String)

            'Create a Document instance
            Dim original As Document = New Document

            'Load a sample Word document
            original.LoadFromFile("E:\Files\SplitByPageBreak.docx")

            'Create a new Word document and add a section to it
            Dim newWord As Document = New Document
            Dim section As Section = newWord.AddSection
            Dim index As Integer = 0

            'Traverse through all sections of the original document
            For Each sec As Section In original.Sections

                'Traverse through all body child objects of each section
                For Each obj As DocumentObject In sec.Body.ChildObjects
                    If (TypeOf obj Is Paragraph) Then
                        Dim para As Paragraph = CType(obj, Paragraph)
                        sec.CloneSectionPropertiesTo(section)

                        'Add paragraph object in the section of original document into section of new document
                        section.Body.ChildObjects.Add(para.Clone)

                        'Traverse through all child objects of each paragraph and determine whether the object is a page break
                        For Each parobj As DocumentObject In para.ChildObjects
                            If TypeOf parobj Is Break AndAlso (TryCast(parobj, Break)).BreakType = BreakType.PageBreak Then
                                Dim i As Integer = para.ChildObjects.IndexOf(parobj)

                                'Remove the page break from its paragraph
                                section.Body.LastParagraph.ChildObjects.RemoveAt(i)

                                'Save the new document to a Docx file
                                newWord.SaveToFile(String.Format("result\out-{0}.docx", index), FileFormat.Docx)
                                index = (index + 1)

                                'Create a new document and add a section
                                newWord = New Document
                                section = newWord.AddSection


                                'Add paragraph object in original section into section of new document
                                section.Body.ChildObjects.Add(para.Clone)
                                If (section.Paragraphs(0).ChildObjects.Count = 0) Then

                                    'Remove the first blank paragraph
                                    section.Body.ChildObjects.RemoveAt(0)
                                Else

                                    'Remove the child objects before the page break

                                    While (i >= 0)
                                        section.Paragraphs(0).ChildObjects.RemoveAt(i)
                                        i = (i - 1)

                                    End While

                                End If

                            End If

                        Next
                    End If

                    If (TypeOf obj Is Table) Then

                        'Add table object in original section into section of new document
                        section.Body.ChildObjects.Add(obj.Clone)
                    End If

                Next
            Next

            'Save to file
            newWord.SaveToFile(String.Format("result/out-{0}.docx", index), FileFormat.Docx)
        End Sub
    End Class
End Namespace

using System;
using Spire.Doc;

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

            //Load a sample Word document
            document.LoadFromFile(@"E:\Files\SplitBySectionBreak.docx");

            //Define a new Word document object
            Document newWord;

            //Traverse through all sections of the original Word document
            for (int i = 0; i < document.Sections.Count; i++)
            {
                newWord = new Document();

                //Clone each section of the original document and add it to the new document as new section
                newWord.Sections.Add(document.Sections[i].Clone());

                //Save the result document 
                newWord.SaveToFile(String.Format(@"test\out_{0}.docx", i));
            }
        }
    }
}

Imports Spire.Doc

Namespace SplitBySectionBreak
    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("E:\Files\SplitBySectionBreak.docx")

            'Define a new Word document object
            Dim newWord As Document

            'Traverse through all sections of the original Word document
            Dim i As Integer = 0
            Do While (i < document.Sections.Count)
                newWord = New Document

                'Clone each section of the original document and add it to the new document as new section
                newWord.Sections.Add(document.Sections(i).Clone)

                'Save the result document 
                newWord.SaveToFile(String.Format("test\out_{0}.docx", i))
                i = (i + 1)
            Loop

        End Sub
    End Class
End Namespace