PEM is a container format that could, among other things, be used to store SSH public and private key pairs. AWS for example, uses PEM format for accessing their Linux EC2 instances.
PuTTY only accepts .ppk (PuTTY Private Key) files, so you will not be able to directly use a .pem SSH private key in PuTTY. However, you can convert your .pem file to .ppk to use your existing SSH private key in PuTTY.
PuTTY Key Generator or PuTTYGen is a free Windows utility that you can use to convert pem files to ppk.
Step-by-step video guide:
Download
PuTTY Key Generator (
puttygen.exe) based on your
CPU architecture from the official website.

https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html
Launch
PuTTYgen.

Click on the
Load button from
PuTTYgen's main interface.

Set the
Files of type to
All Files (*.*).

Select your
PEM file and click
Open.

Enter the passphrase of your
PEM private key file if prompted and click
OK.

Click
OK on the successful import of the file notification.

Edit any key parameters if necessary and click
Save private key.

Click
Yes if you don't set any passphrase on your key before and getting warned about it.

Select the location and filename to save the generated key and click
Save.

Discuss the article:
Comment anonymously. Login not required.