When we download files via internet, we cannot 100% guarantee that downloaded file is completely genuine one without being changed by someone else. Corrupted file or infected file by malware could cause serious security breach.
One way to identify whether the file is original state is to check digital signature if it is coming with it. or you can check the file integrity by checking the hash value. every file has unique data contained in it, and when you apply a certain algorithm called "cryptographic hash function" to it, a string value is returned which is only valid for the file in the current state. If any bit of data in the file is changed, and you apply algorithm again, it will return different value which is different to first value. with this information you can verify this file is the original file or not.
The popular algorithms are MD5 and SHA1 and we see those hash values from product download page. this will give you information to check product integrity after you downloaded by comparing the hash value. There are many free ware tools calculating a hash value using same algorithm such as IgorWare Hasher.