Why are the thumbnails slow on a big document?

Each page is rendered properly so you can actually read it. Documents over about sixty pages render the first sixty to stay responsive.

Can I combine this with another file?

Yes — append a second PDF and its pages join the board, where you can interleave them anywhere you like.

Reorder, rotate, delete and append pages on a thumbnail board, then save.

Runs in your browser — nothing uploaded

Choose the PDF. Every page is rendered as a thumbnail you can see.
Drag thumbnails to reorder, rotate the ones that need it, and mark any page for deletion.
Press Save when the board looks right.

Why are the thumbnails slow on a big document?: Each page is rendered properly so you can actually read it. Documents over about sixty pages render the first sixty to stay responsive.
Can I combine this with another file?: Yes — append a second PDF and its pages join the board, where you can interleave them anywhere you like.

Combine any number of PDFs into one file, in the order you choose.

Pull out a page range, cut a document in half, or burst every page into its own file.

Delete the pages you do not want and keep everything else exactly as it was.

Save chosen pages as a new document, together or as separate files.