LEVEL 1 (4_096 files): [pdf_level1] [pdf_level1] [pdf_level1] ... [pdf_level1]
LEVEL 2 (  256 files): [[pdf_level1] * 16] [[pdf_level1] * 16] [[pdf_level1] * 16] ... [[pdf_level1] * 16]
LEVEL 3 (   16 files): [[pdf_level2] * 16] [[pdf_level2] * 16] [[pdf_level2] * 16] ... [[pdf_level2] * 16]
LEVEL 4 (    1  file): [[pdf_level3] * 16]