Write the order down first, then select files in exactly that order.
Move files up and down until the sequence matches the form.
Rotate any sideways or inverted pages before merging.
One invoice per page, in the same order as the claim form, all oriented for normal reading. The first two come from merging and reordering; the third usually needs rotation, because many downloaded invoices are landscape.
The standard electronic invoice layout is landscape (240×140mm) while claim forms are portrait A4. Merge them as-is and the printed result alternates between portrait and landscape. Normalise everything to portrait A4 with each invoice centred.
It saves a page but frequently breaks the QR code or invoice number on tax verification systems, and you end up redoing the whole batch. An extra page costs nothing.
Reasonable compression will not. If it stops scanning, redo it at a higher quality level.
No — the order has to follow your claim form, so it is manual.
Compress once, or split the batch by month.