There is one user Lets call him B and he needs access Just to Read the File MEANS READ ACCESS and that file is own by another user A .
How can I grant access to user A's file to User B.
That depends on A's group and B's group. If they are in the same, you need to check group permission (middle of the permission line) are enabling reading :
Here, user can read and write, group can only read, and other can only read.
If A and B are not in the same group, then B have the other permissions, and you need to set those to read.
Reminder: to change a file permission, open a terminal (Ctrl+Alt+T) and type