Alright, now that I know what needs to happen, I know what I would do.
This should be a fairly quick and effortless method, even for large grids and even if you have to do this more than once.
However this explanation is going to be very long, because I want to make sure everyone can do it
1. Exporting pages as separate pdf files
- Open Scripts panel in Indesign.
Window > Utilities > Scripts
- Right-click "User" script folder in the Scripts panel and choose "Reveal in finder" ( Or the windows equivalent, what ever that might be )
- Download PageExporterUtility5.0.1.jsx script.
- Export it into that "User" scripts folder you just opened in step two.
- After that it should show up in the Indesign Scripts panels "User" folder.
- Make sure you have the document you want to export open and active.
- Double click the script in the scripts panel.
- The only thing you need to change in the script options is the
- Then just click OK and choose a destination folder.
2. Placing multiple images in a grid
- Make a new document (In the OP's case that would be the A3 size document)
- Make gutters 0, unless you really need to have gutters.
- Choose all of the page pdf files you wish to place.
- Either use "Place" Ctrl+D and use shift or control to select multiple files at the same time
- OR just drag the files to your indesign document straight from the folder.
- Start placing the images like you normally would...
- Drag from the upper left corner to the lower right corner ( Don't let go yet! )
- While still holding down the mouse button, tap the arrow keys to add rows and colums
- Up arrow - Adds a row
- Down arrow - Deletes a row
- Right arrow - Adds a column
- Left arrow - Deletes a row
- If you desired to keep gutters in this document you will see a gutter between the grid sections. Doesn't matter either way, really.
- After you have a desired grid, let go of your mouse button, and the images will be placed in the grid inside their own frames.
3. Joining the rows and columns
- Right-click anywhere in the documents and choose
Fitting > Fit frame to content.
- Select the
- Drag between a row and/or a column while holding Cmd+Alt (mac) or Ctrl+Alt (windows)
- Drag until the rows and/or columns are joined