If your large PDF opens but editing is disabled, then it is most likely an edit restriction or PDF permission lock. Changing PDF readers will not fix the issue. You need a PDF unlocking option, but only use it when the secured PDF is yours or you have approval from the owner. If the file asks for an opening password, get the correct password first.
My suggestion is to check Mailvare Free PDF Unlocker for this type of locked PDF file. It is made for removing print, copy and edit limits and it also supports large PDF files with a preview before unlocking.
Do not touch the main document first. Try the free demo on one sample PDF and check whether the editing restriction is removed properly.
