Is Overleaf limited to 30-pages? The preview seems to stop at that point.
No, you can use Overleaf (formerly writeLaTeX) for documents of any length, and the export to pdf will always contain the full length document. The 30-page limit on the preview pane is only for anonymous users; it is raised to 300 pages if you have signed up for a free account and are signed in.
For our Pro and Pro+ users the preview length is further increased to 1000 pages.
If you have a specific need for a longer preview please get in touch using our contact page.