Why?
1. Maybe you want to email it somewhere (like home).
2. Smaller files are more reliable. Less chance of corruption.
3. They save and open faster.
How?
Obviously, if you've copied the whole model off to the side and don't need it any more, lose it.
UPDATED: Turn off the preference for 'Save Navigator Preview Data With Project File' at Options | Preferences | 2D Redraw Options.
Get rid of extraneous piles of 2D lines. Unlinked S/E's you don't need, copies of views in unlinked S/E's, and 3D views pasted into S/E or Detail windows. To save a 3D view's lines, Save as Module. A perspective with fills can be 10MB easy.
Delete any merged TIFs. Save as Module to hold on to the size/rotation/placement.
In Layers, Hide all except '+Z SE Tool'. On the Display menu, choose Section/Elevations -> Rebuild All Model S/E's from Model'. Not 'All', and not 'Drawing'. That's very important.
This will drastically reduce the size of the file. Why? Because Archicad actually saves the image of each S/E window in the PLN file. That must be so it can put the image up without needing to rebuild, right? Nope, because it does this even for automatic-rebuild sections.
The effect is enhanced if you have the fills on in the sections. I do this every once in a while just for the heck of it, to keep everything neat.
If you are seriously emailing it somewhere, your goal should be to get under 10MB, the limit for many mailboxes. If it's still big, zip in the Finder.
7-22-03
1-26-04
8-27-05
11-30-05