I downloaded Monzo statements from an iOS app just to run through them to check if I recognise all transactions.
One issue I’ve noticed is with how the text is being selected. When you start dragging near the date (first column) and move the mouse right, instead of seeing the current row selected, the entire columns across the whole page get selected. This is quite irritating if you want to copy-paste transactions one-by-one or use a marker in macOS Preview app to highlight a particular transaction.
This issue should be fixable rather easily just by changing the source page markup a bit. Hope your devs will be able to address this next time there is a need to touch things in the PDF statement
In the meantime, try holding down the ⌥/opt/alt key and then make your selection. That allows you to make a rectangular selection that doesn’t have to follow the ‘line’ in Preview (and several other apps). It should allow you to select a horizontal line in the Monzo statement.
Here is a repro in macOS preview. The final selection attempt includes @jzw95’s suggestion to hold opt/alt during selection. It works, but is not intuitive.