Problem with attachment in Apple mail

Update and supplement to previous message

Update:
In addition to what was written before: I can attach a document to an email message if the attachment is in the "Documents" folder.
Address: User / Laurent / Documents

Supplement
I found another problem: When I receive a message with multiple attachments and I want to save the attachments with the "save all" command. I choose the folder where to save the documents, then clic "ok". It seems to work, but then I open the folder and it is empty.
If for the same operation, I choose the "Downloads" or "Documents" folder for destination, everything works correctly instead.
If instead I drag in attachement from the email message to the folder I want with the mouse, instead everything works correctly

The dropbox location is:

Users/**/Library/CloudStorage/Dropbox

To attach documents to Apple Mail I have tried every possible way.
1) Drag and drop the document into the message with the mouse.
2) Click on the button (with icon) inside the message.
3) "File" menu, "Attach file" command.

The current situation is as follows:
1) I can send attachments contained in dropbox only from outlook. With Apple Mail or Spark (Readdle) it is impossible to.
2) I can attach documents to emails (Apple mail and Spark) only if I first move the document to the "downloads" folder.
The location of the folder is : Dropbox / Mac (5) / Donwloads
Even if I move an attachment from a dropbox synchronized folder to the desktop and then attach it to the message, it doesn't work.

Hi there, Laurent N.1, would you mind clarifying the location of your Dropbox folder, as shown in your Sync tab, of your app's preferences? 

If you could also clarify the exact steps you followed, when trying to attach your files, when this issue occurs; that'd be most helpful! 

1) On the computer where I'm having problems with email attachement, backup is turned off.

2) Yes, I see some folders, because in the past I had activated the backup, but now I haven't used it for months.

Did you enabled the Dropbox Backup feature at some point in the past perhaps Laurent N.1?

Do you see any folders being backed up in your backups online? 

Hi Walter

The documents I want to send are offline.

In the meantime, I discovered something really strange: My downloads folder (which is normally located in mac os x under "user") is actually an alias of a downloads folder saved in dropbox (under folder named "Mac5" .

If I move documents (pdf, jpg, png, ...) into this folder and then attach them to an email from this folder, everything works correctly.

But only if I put them in this folder, even though they are offline documents. 

Than you

Hey Laurent N.1 - sorry to jump in here, but I wanted to ask something.

Are the files you're attaching to your emails online only or available offline on your computer when you take a look at their syncing icons locally? 

Another interesting detail: if I want to save received attachments (via the "save attachments" command), I can only save them in a folder called "downloads" which is inside the dropbox folder. If I choose to save the documents anywhere else, even on my desktop, it doesn't save my attachments. This happens with other programs as well.
Exception: if I drag the documents to a folder, the download works (app mail and other software).

UPDATE

I installed "Spark" software (Readdle company), I have the same problems.

It happens with any type of attachment: pdf, doc, jpg, png, etc.

As can be seen in the attached screenshot, only the type of reporting changes. In this case I have tried to attach an image.

Thanks for that! Is this happening with other types of files besides PDFs? Like image files, for example?