Welcome to the Pdfium.NET SDK

PM> install-package pdfium.net.sdk

public void LoadDocument()
{
    //Initialize the SDK library
    //You have to call this function before you can call any PDF processing functions.
    PdfCommon.Initialize();

    //Open and load a PDF document from a file.
    pdfViewer1.LoadDocument(@"c:\test001.pdf");

    //Selects all text contained in a first page.
    pdfViewer1.SelectText(0, 0, 1, 0);

    //Gets selected text from PdfViewer control
    string selectedText = pdfViewer1.SelectedText;
}

<Window xmlns:Wpf="clr-namespace:Patagames.Pdf.Net.Controls.Wpf;assembly=Patagames.Pdf.Wpf"
        xmlns:Toolbars="clr-namespace:Patagames.Pdf.Net.Controls.Wpf.ToolBars;assembly=Patagames.Pdf.Wpf"
        x:Class="ExamplesWpf.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Name="mainWindow"
        >
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="auto"></RowDefinition>
            <RowDefinition></RowDefinition>
        </Grid.RowDefinitions>

        <ToolBarTray Grid.Row="0">
            <Toolbars:PdfToolBarMain Window="{Binding ElementName= mainWindow}" PdfViewer="{Binding ElementName=pdfViewer1}" Name="toolbarMain1"></Toolbars:PdfToolBarMain>
            <Toolbars:PdfToolBarClipboard PdfViewer="{Binding ElementName=pdfViewer1}" Name="toolbarClipboard1"></Toolbars:PdfToolBarClipboard>
        </ToolBarTray>

        <ScrollViewer Grid.Row="1" CanContentScroll="True"  VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto">
            <Wpf:PdfViewer Name="pdfViewer1" />
        </ScrollViewer>
    </Grid>
</Window>

public void ExtractTextExample()
{
    //Initialize the SDK library
    //You have to call this function before you can call any PDF processing functions.
    PdfCommon.Initialize();

    //Open and load a PDF document from a file.
    var doc = PdfDocument.Load(@"c:\test001.pdf");

    //Gets second page from document;
    var page = doc.Pages[1];

    //Gets number of characters in a page or -1 for error.
    //Generated characters, like additional space characters, new line characters, are also counted.
    int totalCharCount = page.Text.CountChars;

    //Extract text string from the page
    string text = page.Text.GetText(0, totalCharCount);
}

public void RenderPage()
{
    //Initialize the SDK library
    //You have to call this function before you can call any PDF processing functions.
    PdfCommon.Initialize();

    //Open and load a PDF document from a file.
    using (var doc = PdfDocument.Load(@"c:\test001.pdf"))
    {
        int i = 0;
        //Iterate all pages;
        foreach (var page in doc.Pages)
        {
            //Gets page width and height measured in points. One point is 1/72 inch (around 0.3528 mm)
            int width = (int)page.Width;
            int height = (int)page.Height;

            //Create a bitmap
            using (var bmp = new PdfBitmap(width, height, true))
            {
                //Fill background
                bmp.FillRect(0, 0, width, height, FS_COLOR.White);
                //Render contents in a page to a drawing surface specified by a coordinate pair, a width, and a height.
                page.Render(bmp, 0, 0, width, height,
                    Patagames.Pdf.Enums.PageRotate.Normal,
                    Patagames.Pdf.Enums.RenderFlags.FPDF_ANNOT);
                //Get .Net image and save it into file
                bmp.GetImage().Save(string.Format(@"c:\test001_pdf_page_{0}.png", i++), ImageFormat.Png);
            }
        }
    }

}

public void ImportPages()
{
    //Initialize the SDK library
    //You have to call this function before you can call any PDF processing functions.
    PdfCommon.Initialize();

    //Open and load a PDF document from a file.
    var docSource = PdfDocument.Load(@"c:\test001.pdf");

    //Create a new PDF document.
    var doc = PdfDocument.CreateNew();

    //Import some pages to a PDF document.
    doc.Pages.ImportPages(docSource, "1,5-7", 0);

    //Saves a copy of PDF document to the specified file name
    doc.Save(@"c:\test002.pdf", Patagames.Pdf.Enums.SaveFlags.Incremental);
}

public void GetAccessToAcroForms()
{
    var forms = new PdfForms();
    var doc = PdfDocument.Load(@"c:\test.pdf", forms);
    //doc.FormFill is equal to forms and can be used to get access to acro forms as well;

    int i = 0;
    foreach (var field in forms.InterForm.Fields)
    {
        if (field.FieldType == Patagames.Pdf.Enums.FormFieldTypes.FPDF_FORMFIELD_TEXTFIELD)
        {
            field.Value = "This is a field #" + (++i);
        }
    }
}

//or if you are using PdfViewer

public void GetAccessToAcroFormsFromViewer()
{
    var doc = pdfViewer1.Document;
    var interactiveForms = doc.FormFill.InterForm;

    int i = 0;
    foreach (var field in interactiveForms.Fields)
    {
        if (field.FieldType == Patagames.Pdf.Enums.FormFieldTypes.FPDF_FORMFIELD_TEXTFIELD)
        {
            field.Value = "This is a field #" + (++i);
        }
    }
}