The main difference between a PQQ and an ITT is the whereabouts in the tender process which it happens. A PQQ (Pre Qualification Questionnaire) is often the first step once the contract notice has been published.
A PQQ is generally a factual document which is required to be completed within a timed deadline. The information which you provide as a potential supplier is then used by the buyer to make a shortlist of potential suppliers. The shortlisted suppliers are then “invited to tender” via a document called an ITT (Invitation to Tender) which usually contains questions relating much more to how the contract will run, how it will be managed and supported etc.
So in a nutshell, the PQQ is the first stage and if you are successful you may be invited to the ITT which is often a more descriptive and thorough document