First, search your subject in sid.ir and copy URL link Then, go to python file, there is information part that include 'url_' and 'pages' and 'adr_' . replace your URL link in 'url_' and enter the number of pages you wanna scripe in 'pages' section and enter the location for saving pdfs in 'adr_ 'section.
url_='https://sid.ir/search/paper/%D8%B1%D9%88%D8%A7%D9%86%D8%B4%D9%86%D8%A7%D8%B3%DB%8C/fa/?page=1&sort=1&ftyp=all&fgrp=all&fyrs=1379%2c1402'
pages = 1
adr_='C:/Users/lenovo/Desktop/BN/web scraping'