I'm trying to place a 90 page pdf (document size is 40x40mm) onto 1 sheet (1 page). Should look like a grid. Is there any way to do this? a script maybe? Thanks.
One could write a script for this, but I think for most designers it would take longer than it takes to click 90 times.
You do not mention which size you want the sheet to have. Should the 40x40 mm pages be in actual size? Or do you need thumbnails at a small size?
Anyway, no need to decide yet. Just start with a big document large enough to hold the pages in their actual size.
90 pages will fit in a 10×9 grid. So if the pages are placed closely together (you don't mention anything about that) we need a page at least 400×360 mm.
Just make a 1000×1000 mm document to have some space to work with.
Then make a 40×40 mm rectangle using the Rectangle Frame Tool (to make it easier to click in), select it and create a grid of rectangles using Edit > Step and Repeat.
Now simply use File > Place, select your file and tick on Show Import Options. Make sure you select All pages and you probably want to set Crop to to Trim.
Click each rectangle to place the pages.
The GIF is real-time (just low framerate to keep it under the 2MB limit), so as you see it doesn't really take that long.
You might notice that I make some mistakes near the end. That is of course entirely on purpose and only to prove a point. 😀 If you make a mistake, don't start over. Just undo that last page and place it again.
After you are done, you can group the pages, scale them to whatever size you want and copy/paste them to another document.