When a PDF document is opened or created, it is stored on disk in a manner that permits direct access to the objects it contains.
PDF document resources are stored in memory as needed, and the memory space is freed when it is no longer needed.
When opening a PDF document, the Adobe PDF Library will only use as much memory as it needs to store the object index. It will not store the entire document in memory. If page 15 has a large image on it and you obtain the PDEPage for that page, the Library will not bring that image into memory. If a page includes a four megabyte image, the Library will only bring that full image into system memory if you image the page, or enumerate its contents, and ask for the content of the image.
How memory is used depends greatly on what the application is doing. The Library will use very little memory when creating PDF documents. The heaviest demand on memory occurs when rendering pages to bitmaps. This requires reading fonts and characters, and rendering characters to bitmaps.