You are making a few assumptions, which may not be valid. It is also plausible that your senior colleague (let's call him John) has already read the critical information and/or the information is not as critical as you think.
Being a Senior Developer, he probably knows better how to assess project requirements than you do. While being a senior doesn't imply that they can never make mistakes, I would be careful about making such an assumption.
Hence, I would suggest that rather than telling him that you found some critical information, you ask him if the information is critical.
John, I was reading through the document, and found this information which seems important, but I do not fully understand its implications. Could you please explain to me how it would affect our project?
If John had already read the important information, you won't look like a moron for telling him something obvious. If John hadn't read about it, he gets to save face.
Note on Edit My choice of the phrase "feign ignorance" led to some misinterpretation, so I rewrote the answer to better describe my intention.
Also, in an ideal world, it is great to make statements like, "if egos are getting in the way of work, there are bigger problems to worry about." In the real world, you will find people with huge egos in the workplace, who are nonetheless extremely skilled, important for the company, and offer a lot you can learn from. You could either refuse to deal with their egos, or you could learn to adapt to it and get your work done.
If asking a senior a question will help you better understand something, there is no harm in hiding your half-baked, possibly incorrect, "knowledge".