Emojis are nothing more than unicode sequences. If you create that unicode sequence in your PDF, then all Apple users will see the character with Apple's emoji.
If you wanted to embed Apple's font, or use Apple's images so that all users of the PDFs see the same exact images, you'll have to review Apple's license for embedding fonts and/or burn some cash with your lawyer.
When I review the "embedding" information for the Apple Color Emoji font, it says:
Preview and print embedding. This font may be embedded in documents and temporarily loaded on the remote system. Documents containing this font must be opened “read-only;” no edits can be applied to the document.
Of course, you would also have to be in compliance with other Apple license agreements, which means your online server will have to be a Mac.
I recommend sticking to the unicode sequences. Then everybody gets the emoji they paid for.