using (var doc = PdfDocument.Load(@"d:\test.pdf"))
{
    var page = doc.Pages[0];
    for(int i=page.PageObjects.Count-1; i>=0; i--)
    {
        if (!(page.PageObjects[i] is PdfTextObject))
            continue;
        var obj = page.PageObjects[i] as PdfTextObject;
        if (obj.TextUnicode == "some text")
        {
            page.PageObjects.RemoveAt(i);
            break;
        }
    }
    page.GenerateContent();
    doc.Save(@"d:\test_modified.pdf", SaveFlags.NoIncremental);
}