Strictly speaking, an epic is a long narrative poem on a great and serious subject. It is written in an elevated style, and has at its centre a heroic or quasi-divine figure on whose actions the fate of many people depends. The setting is huge, often worldwide, or even larger.
Paradise Lost clearly fits the criteria, taking as its subject the Fall of Man, and having the elevated purpose of setting out to 'justify the ways of God to men'. The hero could be seen to be Adam, whose fatal action 'brought death into the world/And all our woe', or Christ, who redeemed that fault, and was both God and man.
Book 9 is in some ways less obviously epic in method and scale than the other books. Milton opens it by declaring a change of mood ('I now must change/These notes to tragic') and the whole book has quite a dramatic quality, with soliloquy and impassioned debate, temptation and recrimination between its protagonists. Milton invokes his heavely Muse, however, to inspire his work and, although the scenes in the garden are more domestic in scale than the earlier books, we still get the cosmic perspective as Satan circles the world, seeking his entrance to achieve his evil purposes. As elsewhere, Milton uses a number of extended epic similes, including a memorable one when Satan observes Eve in her pastoral setting and is, temporarily, almost abstracted from his own evil.