Straight, even light, then crop to the A4 ratio so it matches the body.
Body, appendices, then the signature page for that contract. Repeat per contract.
Print preview at actual size: page count, signature placement, page size consistency.
The body is a 100% A4 export. A phone scan of the signature page usually carries an edge shadow and comes out two to five percent larger, so after merging it prints a visibly bigger page than the rest of the contract. Crop the scan to the A4 ratio before merging, and if the whole contract went through the same scanner, convert all of it at once.
The usual mistake is putting the signature page after the appendices, or after all contracts when several are bundled into one file. The body goes first, then appendices, then that contract's signature page. If you are shipping several signed contracts as one PDF, keep them in the same order you listed them and say so in the covering message.
A handwritten signature is thin strokes. The smallest compression setting breaks them into dots and the signature looks forged or faded when printed. Compress the body only, or keep the whole file at the higher quality setting — a few megabytes is cheaper than a rejected contract.
A picture of a signature carries little legal weight on its own. For anything binding, use the original or a compliant e-signature service.
PDF pages are sequence, not printed numbers. Generate numbering in the source document before export.
Crop to the A4 ratio, or convert it to A4 portrait before merging.