"Pick up something" or "pick something up"?

I have difficulties with word order:

• I have picked up the pencil from the floor. [says my dictionary]
  ?I have picked the pencil up from the floor. [could be?]
• ?I will pick up it. [sounds weird]
  I will pick it up. [sounds right to me]

This from the ‘Longman Student Grammar of Spoken and Written English’:

Where the direct object is a pronoun (1), it is usually placed between the verb and the particle (over 90 per cent of the time) . . . However, when the direct object is an indefinite pronoun (2), it is often placed after the adverbial particle . . . When the direct object is a full noun (3), there is more variation in its placement.

For Example:

1. Pick it up.
2. Pick up something.
3. Pick the pencil up. Pick up the pencil.