1.) It is one of the most important documents in history because this was a pivotal document that not only helped end slavery but also helped the North win the civil war and keep the Union together.
2.) It still would have been seen as a good thing, since the abolistionist main goal was to end slavery. This would have been seen as a start into ending slavery.
3.) Like previously stated, the Emancipation Proclamation was a step into not only freeing slaves but also helping the north win. By freeing all slaves in the South, the South had a very low chance of winning. The slaves did almost everything for them and now the way of life for southerners was gone. The southerners were still infuriated making them want to fight. We all know the end result. Therefore, I think he thought it was his biggest achievement as a President because it was such a smart battle plan.
Hope this helped!