For example you have domain example.tld
with Let's Encrypt certificate name Lets Encrypt example.tld
. You can download certificate file with private key from Plesk UI:

Or you may try to find this certificate files by following request:
# mysql -uadmin -p`cat /etc/psa/.psa.shadow` -Dpsa -e "select id, cert_file, ca_file, name from certificates where name like '%example%'"
+----+-------------+-------------+----------------------------------------------+
| id | cert_file | ca_file | name |
+----+-------------+-------------+----------------------------------------------+
| 3 | cert-1lPfuj | cert-6oXdCf | Lets Encrypt example.tld |
+----+-------------+-------------+----------------------------------------------+
and as @El_Matella said check files cert-1lPfuj
and cert-6oXdCf
at /usr/local/psa/var/certificates
:
# ls -la /usr/local/psa/var/certificates/cert-*
-r--------. 1 root root 5050 Jan 1 02:33 /usr/local/psa/var/certificates/cert-1lPfuj
-r--------. 1 root root 1635 Jan 1 02:33 /usr/local/psa/var/certificates/cert-6oXdCf