The explanation in the link suggests that PF means that the key is part of the Primary Key for the table where it appears as well as being a Foreign Key.
Junction tables often have a composite primary key made up of the foreign keys from the tables they are joining because they represent a unique combination of records from the other two tables.
However it is also possible for junction records to use duplicate combination of the junction FKs plus other fields. In these cases the junction table's primary key would also include one or more of those other fields.
For example a combination of the FKs EmployeeID, TaskID plus a Date. In this circumstance the junction table would indicate PF, PF, PK respectively.