A quick way of doing shell style name matching is using the glob module. Below, I rewrote your code to return a generator of matches from a pdf file. We then add together the counts of all such matches for all the documents.

import os
from glob import glob
import re
from PyPDF2 import PdfFileReader

def search_page(pattern, page):
    yield from pattern.findall(page.extractText())

def search_document(pattern, path):
    document = PdfFileReader(path)
    for page in document.pages:
        yield from search_page(pattern, page)

pattern = re.compile(r'USD')  # Or r'\bUSD\b' if you don't want to match words containing USD

count = 0

for path in glob('//DOCUMENTS/document*.pdf'):
    matches = search_document(pattern, path)
    count += sum(1 for _ in matches)

print(f"Total count is {count}")  # "Total count is {}".format(count)