They are used by the page and the XObjects drawn on it and deal with the overlaying of the colors. These groups are described on page 556 of PdfReference 1.7. 

This happens only in scenarios when the widget has On state appearance defined and Off state appearance missing. If both appearances are missing then RadPdfViewer recalculates the appearances and the checkbox is successfully selected and deselected.

I want to draw the signature box, like in Adobe Acrobat Reader.

This is very important when the document needs more than 1 signature.

Loading a document that contains big images could lead to OutOfMemory exception.

This exception is thrown because there is field name with special characters which are incorrectly modified during the recalculation process of widget appearances. This exception is thrown in the internal RecalculateMissingAppearances method or in RecalculateWidgetAppearancesOld method.

For some fonts the issue is present. Some of them are:

KozGoPro-Medium

KozMinPro-Regular

KozGoPro-Bold

• Create new WPFApplication.
• Add a RadPdfViewer to the main window

<Window x:Class="YourProject.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:MaquetteTelerikPDFViewer"
        mc:Ignorable="d"
        xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"      
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <telerik:RadPdfViewer x:Name="pdfViewer"/> 
    </Grid>
</Window>

• Open the pdf file

    public partial class MainWindow: Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        protected override void OnInitialized(EventArgs e)
        {
            base.OnInitialized(e);
            using(Stream stream = new FileStream(@"radExport.pdf",FileMode.Open,FileAccess.Read)){
                pdfViewer.Document = new PdfFormatProvider(stream).Import();
            }
        }
    }

Images decoded with CCITTFaxDecode filter affect the performance when showing the document.

Most PDF viewers have option (enabled by default) which allows fields to be highlighted with colors predefined in the settings. Required fields are highlighted using different color.

For example: In Adobe Acrobat Reader DC this is enabled in "Edit" -> "Preferences" -> "Forms" -> "Highlight Color" -> "Show border hover color for fields".