E-invoice – Generation of IRN:

No. Invoice no. (e.g. ABC/1/2019-20) is assigned by the supplier and is internal to the business. Its format can differ from business to business and also governed by relevant GST rules.
IRN, on other hand, is a unique reference number (hash) generated and returned by IRP, on successful registration of e-invoice.

IRN is a unique 64-character hash, e.g.
35054cc24d97033afc24f49ec4444dbab81f542c555f9d30359dc75794e06bbe

Yes. IRP can reject an invoice.
IRP will check whether the invoice was already reported and existing in the GST System. (This validation is based on the combination of Supplier’s GSTIN-InvoiceNumber TypeOfDocument-Fin.Year, which is also used for the generation of IRN). In case the same invoice (document) has already been reported earlier, it will be rejected by IRP.
Certain other key validations will also be performed on the portal. In case of failure, registration of invoice won’t be successful, IRN won’t be generated and the invoice will be rejected along with relevant error codes (which give an idea about reasons for rejection.)

IRP will return only the signed JSON. No PDF will be returned. On receipt of signed JSON, it is for the respective ERP or Accounting & Billing software system to generate PDF, if needed.

Upon successful registration of the invoice on IRP, it will return a signed e-invoice JSON to the supplier with IRN and QR Code.