I think telcoM's answer sniffs around the primary cause: It's likely to involve a symlink, just a different one.
Based on my experimentation, there's one set of circumstances that fits the bill:
Test_folder is a symlink to pdf2txt, and
- the OP is using stock macOS
cp, which has a BSD heritage
This sort of symlink defeats macOS cp's cycle detection. (GNU cp, as would've been installed by Homebrew or some other package manager, doesn't follow symlinks by default, even when copying recursively.)
Here's a demo, tested on macOS High Sierra:
$ ln -s pdf2txt Test_folder
$ ls -lAF Test_folder
lrwxr-xr-x 1 aho staff 7 Feb 13 20:13 Test_folder -> pdf2txt/
$ /bin/cp -r pdf2txt Test_folder
cp: Test_folder/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt: name too long (not copied)
cp: Test_folder/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt: name too long (not copied)
# Let's just look at the first 30 lines of a recursive ls
# (And yes, we're looking at pdf2txt)
$ ls -lRAF pdf2txt | head -n 30
pdf2txt:
total 0
drwxr-xr-x+ 5 aho staff 170 Feb 13 20:13 pdf2txt/
-rw-r--r--+ 1 aho staff 0 Feb 13 19:43 test1.txt
-rw-r--r--+ 1 aho staff 0 Feb 13 19:43 test2.txt
pdf2txt/pdf2txt:
total 0
drwxr-xr-x+ 3 aho staff 102 Feb 13 20:13 pdf2txt/
-rw-r--r--+ 1 aho staff 0 Feb 13 20:13 test1.txt
-rw-r--r--+ 1 aho staff 0 Feb 13 20:13 test2.txt
pdf2txt/pdf2txt/pdf2txt:
total 0
drwxr-xr-x+ 3 aho staff 102 Feb 13 20:13 pdf2txt/
pdf2txt/pdf2txt/pdf2txt/pdf2txt:
total 0
drwxr-xr-x+ 3 aho staff 102 Feb 13 20:13 pdf2txt/
pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt:
total 0
drwxr-xr-x+ 3 aho staff 102 Feb 13 20:13 pdf2txt/
pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt:
total 0
drwxr-xr-x+ 3 aho staff 102 Feb 13 20:13 pdf2txt/
pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt/pdf2txt:
total 0
echo, e.g. repeat thecp…line withecho cp…above it, then see if the variables are as expected, and if it repeats unexpectedly.