title: removeFromSuperview then paste new view, it won't release memory
body: <p>I found PDFView will cause memory leak, so I try another way to show PDF.</p>
  <p> I use a scrollView that has a contentView for all the PDF page size. </p> 
  <p><a href="https://i.stack.imgur.com/WNOt0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WNOt0.png" alt="enter image description here" /></a></p>
  <p> For example, if one PDF page resolution is 480 x 320, then 5 pages PDF's contentView size is 2400 x 320. And I reuse 3 UIViews (as above image, preload and current pages) for showing PDF pages on contentView, every page size is 480 x 320. If now it shows 1st page, then preload 2nd page; if it shows 2nd page, then preload 1st page and 3rd page. <br /> 
  <p><a href="https://i.stack.imgur.com/xMoRj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xMoRj.png" alt="enter image description here" /></a></p>
  <p>When scrollview moves to next page, I always load the current shown page and next/last page. So I have to remove page using removeFromSuperview(). Then I load the removed page as new one, and paste it into new position on contentView. </p>
  The problem is, removing the page from superview won't release the memory. So a many pages PDF will crash when memory is overload.</p>
  <p>So how could I force release memory? I have read those old posts but not find the answer.</p>