ConcatPDF is the tool to concatenate PDF files. It can concatenate, extract, encrypt, decrypt, configure PDF files, convert image files to PDF. GUI version and CUI version are both available.
iText.NET is iText porting on .NET Framework by J#. This library allows you to generate PDF, (X)HTML, XML, RTF files on Microsoft.NET Framework including ASP.NET.
FTP client to encrypt/decrypt and upload/download files
Khoorio is a FTP client that allows you to safely encrypt your files with the AES Rijndael cipher before uploading them to a FTP server.
The key used to encrypt the files is derived by a key derivation function (HMAC-SHA1) as follows:
encryptionKey = HMAC-SHA1(hash_plaintext_file,user chosen master password).
With this key the user allows someone to decrypt a specific file, without the need to share his master password to that person.
...This system is designed to be used to exchange messages through networks and servers that can not be trusted.
The client application is a C#.net application that is used as a mail client to send/receive and encrypt/decrypt messages.
The server consists of a few ASP.net pages that handle the message exchange.
I have set up a server to test this with at the address www.crashm.co.uk