This should be easier to read, and cleaner. Three changes:

• Using classic return-type-first syntax for methods. The trailing return type was only added for special cases (where you need to compute the return type after the arguments), not for normal int's and floats.
• Make methods const that do not change the ParameterContainer.
• Make the parameter container copy the local one, rather than the one that's going to change. Make it const to ensure it remains the original PDF parameters.