how do i read a disc with files on it on my im

If this is a CD or DVD and your Mac does NOT have a built-in optical drive, you can use the built-in Remote Disk function to read it from another computer that has an optical drive.

Apple support article here:

Use the CD or DVD drive from another computer with your Mac - Apple Support

Are you referring to a CD or DVD? If so, you will first need an external CD/DVD player if your iMac is a 2012 or later as these models do no come with CD/DVD players. There are many USB players available ranging in price from about $35 to $200 depending on your needs. For example here are two that may suit your needs:

https://eshop.macsales.com/item/LG/P60NS50/

https://eshop.macsales.com/item/OWC/MR3USD24/

A disc is a cd or dvd, a disk (commonly know as a hard disk) is something completely different. Please clarify what exactly you have!

If you have a had hard disk with PDF files on it, connect the disk to the Mac, copy the files to the Mac and open them with the Preview App which is pre-installed on the computer.