While Sitecore would be able to store that many documents, There are some things you may want to consider:
With the Search API in the content editor maintenance will be easier than it used to be, but still managing that amount of documents will not be simple.
e.g there is no way to bulk edit documents, or bulk move documents around. Sure you could use Sitecore PowerShell Extentions for that, but its still not the easiest.
Another potential problem would be the size of the master and web DB's once you have that amount of documents in. This will make backup/restoring and general DB maintenance take longer, disk requirements will go up etc..
Because you are on 8.1 the new publishing service is not available, so if you ever need to publish large parts of the media library, or worst case need to republish the entire site, this will take an extremely long time. Publishing media items is slower than normal items anyway because of the blob data.
With those considerations, if I were looking at storing that many documents and the budget allowed for it, I would look at a dedicated Digital Asset Management system (DAM) - Sitecore have DAM for Sitecore and there are others around. A dedicated DAM will solve a number of the above issues as they are built to manage that many documents.
If you have no budget for that, then Sitecore will be able to handle it, but plan the content tree carefully so you have the min amount of maintenance to do once the documents are there.