Why does referencing a class inside loop cause memory crash

I'm looping through all pages in a PDFDocument (300+ pages) but app crashes with

> Message from debugger: Terminated due to memory issue

The pdf is approx 4mb in size yet each iteration of the loop jumps the memory up approx 30mb. That memory is never reclaimed because the loop ends i get the crash.

@objc func scrapePDF(){
      
        let documentURL = self.documentDisplayWebView!.url!
        let document = PDFDocument(url: documentURL)
        let numberOfPages = document!.pageCount
         
        DispatchQueue.global().async {
          
            for pageNumber in 1...numberOfPages {
              
               print(document?.page(at: pageNumber)!.string!)
              
            }
        }
    }

I found that rather than passing a reference to the `PDFDocument` inside the loop, if instead I create a new instance for each loop this strangely solves the memory issue. I don't quite understand why though. `PDFDocument` is a Class not a Struct so is passed by reference. Meaning it is only created once and then referenced to inside my loop. So why would it cause a memory issue?

@objc func scrapePDF(){
  
        let documentURL = self.documentDisplayWebView!.url!
        let document = PDFDocument(url: documentURL)
        let numberOfPages = document!.pageCount
  
        DispatchQueue.global().async {
  
            for pageNumber in 1...numberOfPages {
               let doc = PDFDocument(url: documentURL)
               print(doc?.page(at: pageNumber)!.string!)
  
            }
        }
    }

Though the above code clears the memory issue the problem with it is that its too slow. Each loop takes 0.5 seconds and with 300+ pages I can't accept that. Any tips on speeding it up? Or why it doesn't give the memory back if referencing the `PDFDocument` from outside the loop

have tried autoreleasepool{} no effect