Verbal Plenary Inspiration
"This means is that the inspiration extends to the very words
themselves—not just concepts or ideas—and that the inspiration extends
to all parts of Scripture and all subject matters of Scripture (plenary
inspiration). There are some people who believe only parts of the Bible
are inspired, or only the thoughts or concepts that deal with religion
are inspired, but these views of inspiration fall short of what the
Bible itself claims. Full verbal plenary inspiration is an essential
characteristic of the Word of God." link:http://www.gotquestions.org/Bible-inspired.html
So, the Scripture of the Bible was perfectly written as God's words to Man through Man by the direct inspiration of the Holy Spirit.
The original documents, which we no longer have, were inerrent in terms of scribal perfection.
However, even though the non-original copies may have scribal errors, God's perfect word of truth is preserved.
That is, in terms of copies, though some scribal errors exist in copied manuscripts, none affect God's essential truth for faith and practice.
Thus, in terms of truth - the Bible as a whole is inerrant and infallible.
How can we know for sure? The indwelling Holy Spirit validates the inspired words of the Holy Spirit.
That is - if you are a bearer of the Holy Spirit, you will also believe in the inerrancy and infallibility of Scripture.
By their fruit you shall know them. Matthew 7:20
Speculation on why God did not preserve the original manuscripts: Man would have worshiped and idolized the objects, thus taking focus from the the primary focus of Man - glorifying God and enjoying Him forever. Instead, we get to see the remarkable Providence of God actively preserving His truth throughout the ages, despite Man's best efforts to destroy it.