Introduction
It happens that there are many standards for storing cryptography materials (key, certificate, ...) and it isn't always obvious to know which standard is used by just looking at file name extension or file content. There are bunch of questions on stackoverflow asking about how to convert from PEM to PKCS#8 or PKCS#12, while many tried to answer the questions, those answers may not help because the correct answer depends on the content inside the PEM file. That is, a PEM file can contain many different things, such as an X509 certificate, a PKCS#1 or PKCS#8 private key. The worst-case scenario is that someone just store a non-PEM content in "something.pem" file.