"No man wished it longer." —Samuel Johnson on Paradise Lost
As a dedicated and fervent Christian, Milton believes that the fall was entirely due to the sin of Adam and Eve. His famous mission with this epic was "to justify the ways of God to man." As he begins book IX, in which the duo sin for the first time, he calls man "disloyal" and speaks of their "revolt and disobedience" (lines 7–8). Firmly on God's side, he describes heaven's reaction: "Now alienated, distance and distaste, Anger and just rebuke, and judgment given" (9–10).
While I wouldn't go so far as to describe Milton's treatment of Adam and Eve as innocent, he does make it clear that they have a formidable adversary in Satan, whose power and wiles do make it a somewhat lopsided battle. Satan is "full of anguish, driven" and "bent on Man's destruction" (62–66). Satan is sketched as rising along with the Tree of Life and searching for the perfect animal to tempt Adam and Eve before settling on the serpent: "In at his mouth the Devil entered" (187–188).
Meanwhile, there is a scene of Adam and Eve laboring idyllically and enjoying conjugal bliss in the Garden. However, Adam is aware of Satan and even warns Eve against temptation: "an enemy we have, who seeks our ruin" (274–275). So even though they know about Satan, know about his plan to tempt them, and know how great they have it, they still fall for Satan's trick, which hardly speaks to innocence. However, this is a somewhat theologically sticky issue, as God, being omnipotent and omniscient, must have known they would eat the fruit, which doesn't seem quite fair.
Finally, what may be a key line in this book is Adam's self-aware statement about man, which is worth quoting in full: "Within himself the danger lies, yet lies within his power; Against his will he can receive no harm but God has left free the Will" (348–351).
Please note: I'm using the 1967 Holt Rinehart edition.