A flatbed-scanned PDF at 600dpi in colour, with the document laid flat on the scanner glass and no creases. If a scanner isn't available, a phone photo taken in bright daylight on a flat surface, perpendicular to the document, no flash, no angle, no shadow. The translator's hardest cases come from angled phone photos under indoor lighting.
The single biggest improvement most applicants can make on old-document translation cost and turnaround is sending a better scan.
The best source is a 600dpi flatbed colour scan saved as PDF. Almost any high street print shop will scan a document to that quality for £2–£5; many UK libraries have public flatbed scanners free of charge. The colour matters because faded ink reads differently under digital colour analysis than under a black-and-white scan, and the translator's screen tools can boost contrast on colour scans in ways that don't work on monochrome.
If no scanner is available, a phone photo on a flat surface, taken in bright daylight near a window with no flash, perpendicular to the page, fills two-thirds of the frame, is usually adequate. The mistakes that produce a barely-translatable file are: indoor light only, phone held at an angle, flash on (which produces a glare strip over key fields), document held in the hand (which curves the surface), close-up of one part of the document (which crops out stamps the translator needs).
For multi-page documents (Chinese notarial booklets, old Pakistani records), scan or photograph every page including blanks. The translator decides whether a page carries content; that decision can't be made if the page isn't sent.