The maximum file size of SQL Server 2012 is 16 terabytes. Assuming an average PDF document size of 500KB, the capacity would be 320 million documents for just a single database. Furthermore, the number of documents is not the major factor in estimating hosting capabilities.
Building and searching through indexes is a time consuming process. Because of this, full text indexing may give some apparent comfort. At the same time, it can overwhelm the storage system with huge index tables. Indexing documents with pertinent but limited metadata or full text indexing confined to a text zone within the document (such as first page or specific paragraph) will ease performance and speed up research.