kman, I used to send Islamabad my additional documents through their ISBADIMMIGRATION @ INTERNATIONAL.GC.CA email and they used to attach it (I never used the DMP Portal Site). After my file was transferred to London, I used both their LDNIMMIGRATION email as well as the DMP Portal ... mainly because when I sent my last proofs from LDNIMMIGRATION email I never received their auto-reply ... then I re-sent those documents thru the DMP Portal and received the auto-reply.
In conclusion, try both methods AS LONG as you get their auto-reply ... because if you don't it means the email you sent them didn't reach their inbox. Personal experience. Hope this helps and best wishes for your PP1 and PP2.