Maybe there is a better way, but how about:

import glob
types = ('*.pdf', '*.cpp') # the tuple of file types
files_grabbed = []
for files in types:
    files_grabbed.extend(glob.glob(files))

# files_grabbed is the list of pdf and cpp files

Perhaps there is another way, so wait in case someone else comes up with a better answer.

There's going to be almost no difference. Under the covers, glob is essentially doing the same thing as your listdir code (ie calling listdir and then checking if the paths match a particular pattern). In fact, it's actually doing a bit more work since it's more general (ie. handles more types of patterns than just "end of file"), so it's actually going to be the slower one.

However the difference is going to be pretty much undetectable unless you're looking at directories with millions of files (and even then is going to be pretty negligible). Computers are fast, and this kind of check is going to take a tiny amount of time - the time to actually access the disk is always going to completely dwarf it - trying to optimise it is like trying to reduce the time it takes a train door to open in an effort to cut down the time a 10 minute journey takes. Sure: maybe you could cut half a second off the time with a faster door, but it's not going to be noticable next to the travel time.

however I need to specify a chdir() before it starts

Not at all - you can include the path as part of your pattern. Ie glob.glob("path/to/file/*.pdf")

There are a couple of ways:

pathlib.Path().rglob()

Use pathlib.Path().rglob() from the pathlib module, which was introduced in Python 3.5.

from pathlib import Path

for path in Path('src').rglob('*.c'):
    print(path.name)

glob.glob()

If you don't want to use pathlib, use glob.glob():

from glob import glob

for filename in glob('src/**/*.c', recursive=True):
    print(filename)   

For cases where matching files beginning with a dot (.); like files in the current directory or hidden files on Unix based system, use the os.walk() solution below.

os.walk()

For older Python versions, use os.walk() to recursively walk a directory and fnmatch.filter() to match against a simple expression:

import fnmatch
import os

matches = []
for root, dirnames, filenames in os.walk('src'):
    for filename in fnmatch.filter(filenames, '*.c'):
        matches.append(os.path.join(root, filename))

This version should also be faster depending on how many files you have, as the pathlib module has a bit of overhead over os.walk().