“On paper” literally means “on paper,” but in actual use it means “theoretically,” “superficially,” or “on the surface.”
In other words, it is an expression used when something looks good from the outside but is different in reality.
For example, when a résumé or proposal looks perfect on paper but is not really that way in practice, you can say, “On paper, it looks great, but...”
It is a common expression in business, academic, and everyday English.

3 everyday English examples
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It looks perfect on the surface. → On paper, it looks perfect.
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He’s a great candidate on paper. → On paper, he’s a great candidate.
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It works in theory, but it will be difficult in real life. → On paper, the plan works, but it won’t in real life.
Examples of academic/professional English expressions
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In theory, this approach is the most efficient. → On paper, this approach is the most efficient.
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The study design was perfect on paper, but the actual results were different. → On paper, the study design was flawless, but the outcomes differed.
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The protocol seemed safe on paper, but there were unexpected adverse effects. → On paper, the protocol appeared safe, yet unexpected adverse effects occurred.
In this way, “on paper” is very useful when expressing the gap between theory and reality.
It is especially often used in conferences or presentations when naturally contrasting “in theory, yes, but in practice...”.