No such thing as an owners manual for military surplus rifles with the exception of a few and the Spanish 1916 isn't one that I'm aware of.

The 1916 mauser is a small ring mauser which I expect you learned in your research. As such, it is not considered to be strong enough to handle the 308 or Nato round even though many have been converted as such.

Many people obviously own them and shoot them with no ill effects. Will it blow up? Probably not but over time it will stretch the receiver to the point that it will no longer function properly or will split the cases releasing a stream of hot gas. This could be thousands of rounds so it won't happen overnight. If you are a casual shooter, you probably have nothing to worry about but you should examine the spent cases very carefully after each session. If they are stretched excessively, cracked or have excessive powder burns traveling around the neck, you are getting some warning signs. Most cases stretch on firing so it's a judgement call.

Recommendations are to hand load and keep the pressures in the range that the receiver was designed for. Then you have no unusual safety issues to be concerned with.