There is not a resolution as such of a PDF. PDFs are containers for other objects. Graphics are one of those object types. The resolution of the graphic object is defined by the object, unless the resolution was reduced upon the PDF creation.
However, it seems like you may be wanting to change the size, not the resolution. A general change in size is not a normal function. A back door way (not normally recommended) is to print to a new PDF and select to expand to fit.