After purchase, you will receive the following files, which include the key, intermediate certificate, and more:
The useful files here are the key + site certificate + intermediate certificate, namely domain.key, SSL Certificate.cer, and Intermediate Certificate.cer.
Open these three files separately, copy their contents, and paste them into a new file (separate each file’s content with a line break). Save it and change the new file’s extension to .pem, and you’re done.
