First, convert your textbox entry to a number using TryParse:
double result;
if (!double.TryParse(ArtikliKolicinaTextBox.Text, out result))
{
... report problem to the user - bad number entry ...
return;
}
Then you can output it without any decimals either with a Round method, or (simpler) by casting it to a integer:
pdfDoc.Add(new ListItem($" Quantity: {(int) result}"), calibri10bold));