There seem to be two parts to your answer. Firstly, when the Excel is converted to a PDF, all the layout and all the formula information is lost. It’s only slightly better than a photo. For example, the PDF stores the location of the blocks of text on the page - with no data about rows or columns. There are several software suppliers (including ourselves - pdf.zanran.com) who produce software to reverse engineer back from the PDF to Excel. Secondly, even when a PDF is completely locked down you can always print, scan and OCR it. But you’ll need to check that the letters and digits are absolutely correct. If the scan quality is poor and the font size small, you can get 3 seen as 8, a comma seen as a stop, etc.
Let's assume for example that the data is encrypted using a one-way function by the company. It would be impossible if someone were to come back to the corporate computer and change the encryption keys on the data. Let's suppose that when you enter one-way encryption the first byte of the plain text (which is the password's plaintext) is changed by the company. So... yes, there is a way to crack the encryption key if the person who wrote the encryption file already knew how a one-way function works. But how is it possible that a company uses 1-way encryption on all files it stores? Well, there are several reasons why one would want to use 1-way encryption: Data security. The plain text of the file should not be able to leak the information. The encrypted information should not be accessible to unauthorized users. Availability. One-way encryption.