CONFIGURACAO 300 DPI

I agree that this is "just a metadata value", however, if there are a specific number of pixels, then the wrong PPI value will result in the file being smaller or larger print/physical size when placed in Illustrator or InDesign. Yes, the image can be scaled to a different size in layout and the effective PPI will change accordingly. So depending on one's perspective, it may or may not be a big deal.

If the files come from ACR or Lightroom, both have 240 as default. You can change that in Camera Raw Preferences and Lightroom Preferences (not Photoshop preferences).

As c.p. says, it's not really important. It does not affect the file itself, it's not a native property of the image. It's just a printing instruction stored as metadata.

Not sure what you mean. 

Could you please post screenshots to clarify? 

If you set the Resolution to 300ppi in Image > Image Size (without Resampling) then it should stay that way after saving or Save a Copy. 

Exporting may strip that data, but as it is pretty much meaningless in and of itself that usually hardly matters.