@Moc, I will try, if I can, maybe I got this wrong.
He is in tinymce edit field. He is writting text. He wants to add link to file to some text. If you use Add url button, you need to know exact URL adress, but that file is not on server yet. So you want to upload it. If you use unbuilt media manager, you can upload pdf file but it is uploaded to Page images media category - or something like that (I consider this as bug, this shouldn't be allowed) - you can check this in MM, file is there with wrong category. But you can't select it (you don't see it), because it is not image.
In WordPress you have under Tinymce button Add media file, after click you add any allowed media (you go to Media manager) or select anything from MM, and after selecting it's inserted as link to your body if it is pdf file (I don't use this way, but it works). With e107 you have limitation for each bbarea field. See my issues about this topic.
Problem is that there is no option upload pdf file (only image and video) and it depends on settings for edited field. If you do it manually with MM, it works (common files category is used).
Tinymce 3 had option to upload file to server and select it (you had to allow this in settings). Tinymce 4 doesn't have it (you can't allow it because it's paid feature). CKE editor still allows this.
Added: With dowload plugin you get URL adress for each download, yes, I would use it too, because it's good for SEO and you have overview for all files you put to download, but for so simple task it's too complex. Not mention access, but if you don't want so much new urls (it's not only one link)... shortly, problem is somewhere else, not in download plugin.