Should I create separate table or have the information as columns in the same table
If you really want to normalize your design, here is a suggestion to do so. You should be able to create foreign key constraints. This is without the extra Instruction table since all documents will have the instruction details, stored in the Document table:

Executor
----------
ExecutorID


Document
----------
DocumentID
ExecutorID
Deadline
Target
-- other columns, relative to all documents


DocumentA
----------
DocumentID
-- other columns, relative only to documents of type A


DocumentB  
----------
DocumentID 
-- other columns, relative only to documents of type B


and their relationships:

              Executor
                /1
               /
              /
             /0..N 
         Document
         1/    \1
         /      \
        /        \
   0..1/          \0..1
 DocumentA      DocumentB


*Note: the only constraint not shown (and enforced) with the above design is that a document has to be of either A or B type (and not both.) This can be enforced as well, with a somewhat more complex design.