Why should I use Add Attachments and not a hyperlinked field? What are the advantages in case of a lot of layers with a lot of attachments at the GDB? What are the best way to manage attachments for a large database that contain a lot pf layers with attachments (JPG, PDFs, DWGs) in term of the paths (pre stage before publishing to ArcGIS Server)?
When you use attachments, the source files are imported into the geodatabase and are stored as binary objects (BLOB). This makes it possible to store your related data centralized. You have also much more options for automating the process of importing related data, such as with the Generate Attachment Match Table GP tool which will let you go through the files' names in your workspace and match them to the features/rows based on a certain name. Hyperlinks are a bit more difficult to manage and to my knowledge there are no GP tools for maintaining the links in a good way.
If you are not able to store the related data within the geodatabase and the related data will be always accessible - use hyperlinks. If you want to store all the related data within the geodatabase and make it accessible offline without having the connection to office or if you plan to automate maintenance of the attachments (when new attachments coming all the time) - use attachments.
Another big advantage of GDB attachments, you can have a one to many relationship with them. With hyperlinks, you cant (not easily anyhow). However, in short, database storage is more costly than flat file storage, but has benefits such as replication and easier backup strategies.