You’ve tried all the variations except the one that works. The $ goes at the end of the pattern. Also, you’ll want to escape the period so it actually matches a period (usually it matches any character).
r1 = re.compile(r"\.pdf$")
However, an easier and clearer way to do this is using the string’s .endswith() method:
if filename.endswith(".pdf"):
# do something
That way you don’t have to decipher the regular expression to understand what’s going on.