Using C# PDF SDK
C# PDF Library: how to add, remove, update PDF page background using c#

String inputFilePath = Program.RootPath + "\\" + "1.pdf";
String outputFilePath = Program.RootPath + "\\" + "1_backgd.pdf";

//  open a PDF file
PDFDocument doc = new PDFDocument(inputFilePath);

PDFBackgroudRes resBackground = new PDFBackgroundFillColorRes(Color.Yellow);

{
    //  define a page background setting
    PDFPageBackground background1 = new PDFPageBackground(resBackground);
    background1.Opacity = 0.2F;

    //  define page range: all odd pages
    PageRangeOptions pageRange1 = new PageRangeOptions();
    pageRange1.AllPages = true;
    pageRange1.Subset = PageRangeSubset.Odd;

    //  apply page background settings to all odd pages
    PDFPageFieldHandler.ApplyBackground(doc, background1, pageRange1);
}
{
    //  define a page background setting
    PDFPageBackground background2 = new PDFPageBackground(resBackground);
    background2.Opacity = 0.2F;

    //  define page range: all even pages
    PageRangeOptions pageRange2 = new PageRangeOptions();
    pageRange2.AllPages = true;
    pageRange2.Subset = PageRangeSubset.Even;

    //  apply page background settings to all even pages
    PDFPageFieldHandler.ApplyBackground(doc, background2, pageRange2);
}

doc.Save(outputFilePath);

String inputFilePath = Program.RootPath + "\\" + "1_backgd.pdf";
String outputFilePath = Program.RootPath + "\\" + "output.pdf";

PDFDocument doc = new PDFDocument(inputFilePath);
PDFPageFieldHandler.RemoveBackgrounds(doc);
doc.Save(outputFilePath);

String inputFilePath = Program.RootPath + "\\" + "1_backgd.pdf";

PDFDocument doc = new PDFDocument(inputFilePath);

//  get all Background settings in the document
PDFPageBackgroundInfo info = PDFPageFieldHandler.RetreiveBackgrounds(doc);

//  get default setting for Background and Page Range
PDFPageBackground defaultSetting = info.GetDefaultSetting();
PageRangeOptions defaultPageRange = info.GetDefaultPageRangeSettings();