Question - If your study permit is about to expire, and you cannot apply for PGWP due to delay in obtaining the required documents.

Change your status to Visitor Visa to keep a valid status
ANS - Make sure to do this to avoid complications when applying for your Post - Graduation Work Permit (PGWP).
However, if your study permit expires, before you apply, here are two options you can go for...
(1) Restore your study permit status before applying for PGWP.
You have up to 90 days after your study permit expires to restore your study permit status.
(2) Leave Canada, then apply for a PGWP permit from outside Canada. You've 180 days. From your study completion date to apply.
What if your study permit expires after you apply for PGWP?
You can continue working in Canada as a temporary resident until a PGWP decision is made.