What's the point of sending a file if it can't be modified? As stated by padlofski, there are many ways to change it without your knowledge. For example, we have turned over mylar plans to a client who then erased things and changed the design and bid the job without our knowledge of the change(s). That client then complained that the project as constructed had design flaws! The problems were a direct result of the unauthorized changes made to the plans. What are you going to do? Sue your client?
1) project archival - just burn a CD and send that
2) If you wish to allow the client to "check your work - just print it out or send a scanned image and include the equations so it can be checked.
3) Always put your stamp or signature on the original printout or on the attached transmittal
4) make sure you put a disclaimer on all digital media and preferably on plans and reports also. An example is shown below:
DISCLAIMER & COPYRIGHT ACME CONSULTING INC.: ACME Consulting Inc. assumes no responsibility for data supplied in electronic format. Such data is provided for convenience only and the recipient accepts full responsibility for verifying the accuracy and completeness of the data. The original hard copy of the data, which has been sealed and signed, shall constitute the official documents of record for working purposes. In the event of inconsistencies between the electronic data and the original hard copy data, the hard copy data shall prevail. The recipient releases ACME, its officers, employees, consultants and agents, from any and all claims arising in any way from the content or provision of the data. Nothing herein shall reduce or diminish ACME's ownership of or copyright in the data or its compilation or arrangement. Any analyses, programs, systems, software or formatting in the data shall be the property of ACME. The recipient of this data is prohibited from redistributing and from using any design or drawing information contained within the data, in whole or in part, for any other purpose than that for which it was originally designed without the express written consent of ACME Consulting Inc.