Welcome to the forum!
The feature I like the most is that it can detect the font use inside the pdf and propose a similar one. It's a shame it doesn't work for image....
I'm not sure what you mean. You want Inkscape to propose a similar image....orrr....?? Oh! Do you mean you want Inkscape to propose a similar font if the text has been converted to paths? Yeah, I can see how that would be nice. Although I don't know how it would be possible. It would have to be something like facial recognition I guess, haha!
....make a stroke with 
then step 2 is to right click it/select edge and shape then step 3 go to edge shape's tab and finnaly step 4 : use the line "marker" to make arrow.
I'm not clear what you meant about "select edge and shape" and "edge shape's tab". I can't find anything about edge or shape in the context menu, but maybe there's a confusion with translation. Anyway, here's a shorter route.
1 - draw the path with Pen or Pencil tool
2 - Object menu > Fill and Stroke > Stroke Style tab > Markers
It should take 2 or 3 clicks (after you draw the path), depending on whether you might already have the Fill and Stroke dialog open. It's kind of a central dialog for using Inkscape, which a lot of users probably keep open most of the time (or at least docked).
Yes, I agree with Moini that Inkscape's primary purpose is for drawing, rather than drafting or CAD. For some reason, a lot of people expect Inkscape to work like CAD programs, and are surprised that it doesn't. I'm not quite sure how they have that impression. But I guess Inkscape can be handy for a LOT of different professions and avocations and fields of study, and users' impressions come from their own unique needs and perspectives.
And I was wondering if there is a feature to make this function into an object like this automatically?
I think there is an extension which will do that. And as Moini mentioned, a new LPE is under development. I'm not sure if it might be available in a development version yet?
Ok, here's the extension:
https://github.com/Rutzmoser/inkscape_dimensioning And here are instructions for installing it:
https://inkscape.org/en/learn/faq/#how-install-new-extensions-palettes-document-templates-symbol-sets-icon-sets-etc I don't know whether it actually works with your version of Inkscape. There is another relatively recent message about this, with another good suggestion, but I don't remember the suggestion. But if you search the forum for "dimension" you'll probably find it.
My third remark is about a problem that I didn't solve. I work generally with a pdf that I drag and drop on inkscape then I modify it. The problem is when I save as a new pdf a particular file I had only the modifications without the drawing I input ... I read it can be a problem about the path but even when I redid it It didn't work.
Any idea?
It's hard to say why the original PDF content does not show up when you save as PDF. We would need to either see the PDF in question, or at least know all the details about its contents. There are some bugs with PDFs which I can't recall all of, at the moment. It seems like recently I read where pattern fills don't always show up. And there might be other type of objects which don't convert properly. If the entire contents of the image are not inside the page borders, that might be another reason why it doesn't show up.
For exporting the PNG, the image quality should be quite fine, unless you scale or re-size the PNG later. If you resize any raster image (or zoom it in an image viewer), it will become pixelated. But the original PNG from Inkscape, no matter which DPI you choose, should be fine.
Hhmm, not unless the SVG (or PDF which is opened in Inkscape) contains a raster image. If it contains a raster image, and you export it to PNG at a different dpi from the existing raster image, that image would probably become pixelated.
Yes, a lot of users, including me, are quite confused about DPI and resolution, both in general, and in how Inkscape uses it. 96.0 is the default, and as long as you use 96, the size you set for your image or object or drawing is exactly the size that will be exported.
If you need a PNG with a certain DPI, set that DPI in the Export PNG dialog. Yes, your image will get larger (or smaller). But when you're ready to print it, that's when you set the size or dimensions you want for the printed image. Then when it's printed, it will have the right size and the right DPI.
