while True:
        if  space.free > 3500000:

while space.free > 3500000:

import shutil

space = shutil.disk_usage(r"z:")

num = 1

while True:
    if  space.free > 3500000:
        filemame = "Test_PDF " + str(num) + ".pdf"
        shutil.copy(r"C:UsersMattDesktopTest_PDF.pdf", r"Z:" + filemame)
        num += 1
        print(space.free)