Modeling a database for a printing company
This part in Item doesn't look right:

F.K = JID, OID
  JID REFERENCES PURCHASE (JID)
  OID REFERENCES PURCHASE (OID)


I suppose the design is only on paper, as no DBMS would allow that, when the primary key of PURCHASE is (JID, OID). You need one (composite) Foreign Key constraint, not two: 

  (JID, OID) REFERENCES PURCHASE (JID, OID)


Now this foreign key belong to Puschase_Materials (or Purchace_Items) and not to Materials (or Item). With that in mind, the corrected schema would be:

PurchaseOrders (JID, OID, ODate)
PK = (JID, OID)
FK1 = (JID) REFERENCES Jobs (JID)

Items (IID, Stock, Description, Price)
PK = IID

Purchases_Items (IID, JID, OID)
PK = (IID, JID, OID)
FK1 = (IID) REFERENCES Items (IID)
FK2 = (JID, OID) REFERENCES PurchaseOrders (JID, OID)




The image shows an additional relationship, between Purchase_Materials and Jobs/PrintingJobs, which doesn't appear in the schema - and you don't really need it.

Since there is a foreign key from Purchase_Items to PurchaseOrders (or Purchase_Orders in the image) and a foreign key from PurchaseOrders to Jobs, you don't need a direct foreign key. Consistency is enforced through this indirect relationship.




There is inconsistencies in the naming, between the schema and the images. I assumed the following are renames:

Schema         - Image              - Code in this answer
---------------------------------------------------------
Jobs           - PrintingJobs       - Jobs
PurchaseOrders - Purchase           - PurchaseOrders
Item           - Materials          - Items
  (none)       - Purchase_Materials - Purchases_Items

Some table names are plural and others singular.
Some names have underscores between name parts, some not. 


It would be best if you fixed those and decide on a naming convention (say all table names are plural, there are no underscores except in case X, etc).