The export worked, and the file is hundreds of megabytes. It is slow to open, slow to send and slow to print.
Why it is big
A standard PDF export renders every tile as an image. The size is roughly:
sheet count x sheet area x resolution squared
A 30-sheet poster at Max (600 DPI) is an enormous number of pixels, and there is no compression clever enough to hide that.
Fix 1 — lower the export resolution
The most effective change by a distance, because the relationship is quadratic. Going from Max (600) to High (300) is roughly a quarter of the data; going to Standard (160) is roughly a fourteenth.
Standard is plenty for a poster viewed from a couple of metres. Reserve Max for line art and fine type. See export resolution.
Fix 2 — export as vector PDF, if your content suits it
A vector PDF export does not render text, SVG art or imported PDF pages to pixels at all. For a poster made of type and vector graphics the file can be dramatically smaller and sharper at the same time.
It also avoids duplicating content across tiles: a 6 x 6 poster references the same embedded artwork 36 times rather than storing 36 copies.
The catch: photographs are raster in any container. A photographic poster gains little, and the app warns you before a vector export in exactly that case. Pro feature.
Fix 3 — reduce the sheet count
Fewer, larger sheets is less total paper area to render. A larger paper size is the direct lever — see more sheets than expected.
If you only need to send part of it
Export as images and send the tiles that matter. It also avoids the "attachment too large" problem entirely, since each file is a fraction of the whole.
If a print shop rejected the file
Ask what they want. Most take a standard PDF at 300 DPI happily, and many prefer it to a vector file. Tell them explicitly to print at 100% with no scaling — see print settings that matter.