No difference between the 300 dpi file and the 100 dpi

"I am aware of that method, but, in my case I open two different file with the same content but different resolution and there was no different between the two"  Yes, that's exactly right. If they have the same pixel count. You haven't mentioned pixel count.

DPI is something that seems to cause more confusion with image size that any other aspect.  As Ged has said, DPI relates to the size a document will print.

This image is 800 pixels wide, and has a pixel density (DPI) of 72.

I have the rulers set to inches, so 800 ÷ 72 = 11.11 which is reflected in the ruler.  The image will print at 11 inches with these settings.

However 72DPI would not make a good print, and we tend to think of 300DPI as optimum.

So if I change the DPI to 300, but uncheck Resample then the image still has 800 pixels wide but will now print at just 2 inches wide.

Remember, this is exactly the same image.  The only thing that has changed is the information sent to a printer telling it what size to print the image, and how thinly to distribute the available pixels.

If we left Resample enabled and changed the image size, then we generate more pixels

The image has gone back to printing at 11 inches wide, but the DPI is now 300, and it is now 3333 pixels wide.

But at what cost to image quality?

This shows the image viewed with 1:1 zoom ratio and it looks pretty damn horrible (Double click the image to see it full size)

This is more or less the same area at its original resolution.  It is smaller, but its quality is much better.

Does this help?

Forget ppi. Count pixels. If something worked before it was because you luckily set the right number of pixels.

i know that...originally I needed to difference size for the file i am working on, so i purposely created the file with 300 dpi, its not the best way to do that but it worked before...

No, there's not supposed to be. Lots of people misunderstand pixels per inch (ppi, not dpi). A raster image file is just pixels, so many pixels high by so many pixels wide. It's only pixels. There is no ppi in the file - it's added (arbitrarily) later as metadata.

Pixels per inch is a print parameter, it does not apply on screen. When a raster file is displayed, the image pixel grid aligns to the screen pixel grid, in a 1:1 ratio. One image pixel is represented by one screen pixel - that's Photoshop's 100% view. (If the image doesn't fit on screen because it's too large, it's scaled down in the video pipeline, but that's a different matter).

In other words: on screen you already have a fixed resolution - namely the screen resolution. The image aligns to that.

Here's the thing: On paper you don't have this existing pixel grid to align the image pixels to. So one has to be invented. That's the ppi figure! How many pixels to stretch out on one inch of paper, or in other words, how big the pixels need to be printed to fit the desired number of them into one inch. That's print resolution, ppi.

Ppi is also used for some secondary purposes, such as calculating font sizes in a document. This is because font size is a physical size unit, and it needs to be translated into pixels to have a meaning in Photoshop. This is why Photoshop always assigns a ppi value even if there isn't one already. Any ppi value will do, it can be changed arbitrarily later without affecting the pixel data.

Certainly, yes. The ppi is a piece of information that might be used in printing. It is not a guide to quality or a different kind of image.

It is your job to make sure there are enough pixels for the task you have to do. And don't check quality by zooming above 100%. No, never!