The full document must be visible on the scan or photo, with nothing covering it.
Give each file a descriptive name, such as "bank statement 2019".
Check everything before you submit: you cannot upload more evidence afterwards.
How to do it with Requisito Listo
Open «UKVI: upload visa evidence».
Scan the pages with the camera or pick a PDF or photos you already have.
Put the pages in order.
The app builds the PDF under the size the portal asks for.
At the end you'll see a checklist: size, file weight, background and format. If something doesn't comply, it tells you what to fix before the portal rejects it.