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When you see -1 as the value for a VARCHAR or NVARCHAR data type, that is just a numeric representation for MAX for the .NET SQLDBTYPE as stated here.
So it looks like the application probably just stores the serialized encoded string representation of the PDF in the database in a VARCHAR(MAX) column then.
I assume the developer who designed this thought it was the easiest way to store the PDF, as opposed to VARBINARY(MAX), which would save you space (as Dan Guzman notes in the comments). Additionally it's actually best practice to not store files themselves in the database, but rather on a file system somewhere, and store the location reference to that file in the database instead.