How can user(external) upload some specific document(media-file ex- PDF file) to CD server?
Below point need to take care: Uploaded media files should not be overridden once publishing is done.So media files must be also maintain onto CM(master db). Here also CM user can manage uploaded files.
Here we are using 3 CD server(load-balancing) and 1 CM server.
Sitecore 8.2 update 3.
Other option is to add other Sitecore Web Database like UserGeneratedContent DB and you add you media file there. You need to configure this database exactly like your current web database.
This database is not added to the Publishing Target databases ( /sitecore/system/Publishing targets). In this way you avoid to override content from master database when you publish media files.
In the CM server you will have a scheduler task which will run from time to time and you get the media files from UserGeneratedContent DB and you save it into master DB.
I have done this previous by defining a custom Sitecore event adding it to the event queue. the CM server can then respond to the event by adding the uploaded image to the media library and publishing.
The following link gives a good rundown of how to create and queue custom events. https://sitecore-community.github.io/docs/pipelines-and-events/events/
I would suggest writing an Azure Function that handles media uploading, and then put that back into the master database via a service call to the CM instance.
