FileName="1.pdf",filepath="f:/test1"
FileName="2.pdf",filepath="f:/test2"
FileName="3.pdf",filepath="f:/test3"
FileName="4.pdf",filepath="f:/test4"
FileName="5.pdf",filepath="f:/test5"
FileName="6.pdf",filepath="f:/test6"

var reader = File.OpenText("f:/test5");
reader. // ?

string filename =
   Path.Combine(
       Environment.GetFolderPath(Environment.SpecialFolder.DesktopDirectory),
       "Test1.txt");

string[] lines = File.ReadAllLines(filename);
var list = new List<string>(lines);
list.RemoveAt(4);
File.WriteAllLines(filename, list.ToArray());

int index = list.FindIndex(s => s.EndsWith("/test5""));
// Or whatever test is appropriate.

if (index >= 0) {
    list.RemoveAt(index);
    File.WriteAllLines(filename, list.ToArray());
}