[Geoff Bacon]

Re "the previously loaded PDFs have to be cleared out of memory", I thought you were just storing the blocks in a garbage area i.e. you could clear them on their next reuse (perhaps have a flag so no need to clear them if just allocated). If no obvious space for a flag, you could use the first few bytes in the buffer to store a signature when the block is no longer required (if the signature is present then the buffer needs to be cleared, if zeros then no previous use) 
Historically, the VAX/VMS operating system could develop excessive paging if one iterated through a large array in a non-optimal manner, I doubt that you have this problem (Vax only had 512 byte pages!)

Geoff