存疑 is a verb-object compound (离合词) literally meaning 'preserve doubt'. It is used formally to indicate that a matter or question is left open, not settled, or that judgment is suspended. As a separable verb, you can split it by inserting a number, measure word, or object between 存 and 疑, e.g. 存了一个疑 ('left a doubt'). It is also commonly used as a noun meaning 'existing doubt' or 'unresolved question', though the POS given here is verb.