There are several ways to share files in Linux, including FTP, SSH, Samba, NFS
If you want to share files and keep them in sync in both machines, probably Samba and NFS are the best options. If you only want to transfer a file from one machine to other probable FTP or SSH would be the simplest.
See the following docs for setting them up:
FTP (on the server side):
sudo apt-get install inetutils-ftpd
SSH (on the server side):
sudo apt-get install openssh-server