Combine a scan of the fronts and a scan of the backs into one PDF, alternating. Everything happens in your browser.
This tool runs entirely in your browser: your PDF is not sent to a server. A password-protected PDF will not open here; remove the protection first with the Unlock tool.
Good to know
A scanner without double-sided scanning does the stack twice: first the fronts, then you turn the stack over and scan the backs. The last back then comes in first. With 'reverse order' switched on, the tool puts them back: the last page of the backs belongs to the first front. If you turned the stack over along the short edge, the backs are upside down; then also switch on 'rotate 180°'. The preview shows which page goes where, before you save.
If there is one back fewer, the last front gets no back. If the difference is larger, whatever is left over goes at the end and the tool tells you. If you scanned everything into one PDF, first the fronts and then the backs, choose 'One PDF': the first half are the fronts. If some backs were blank, remove them afterwards with Remove blank pages. Everything happens in your browser; the scans are not uploaded.