Fortunately for you, WhatsApp actually has a built-in option for this. No need for third party apps!
WhatsApp has an option called "email chat" which converts your chat into a word document containing all conversation with a specific person or group.
Simply open WhatsApp, press on the conversation you'd like to backup. Open the menu on top right, press on "more" ->" email chat" it will ask you if you want to include your media or not. It will then bring you to your email app where you can select a recipient. You can choose yourself as the receiver. After you receive the email, You'll be able to open the conversation as a word document.
Depending on the app you use to view word documents, you should have an option to save as PDF.
This is how it will be displayed in your word document:
2/2/18, 03:55 - smith: text
2/2/18, 03:55 - jack: text
2/2/18, 03:56 - Smith: text
Emojis will be included in the word document. Media and voice notes will be sent in a separate file.