Open your document and click on the file menu, then click on Save As. On the right hand side you will need to choose a location to save the password protected version of the PDF. Now drop down the Save as type box and choose PDF. This will make an options button appear, click on it.
To do the math, each base pair takes 2 bits (you can use 00, 01, 10, and 11 for T, G, C and A). Multiply that by the number of base pairs in the human genome, and you get 2 * 3 billion = 6,000,000,000 bits. And remember, you have to go from bits to bytes to get to an answer in megabytes. A bit is just a single unit of digital information, but a byte is a sequence of bits (usually 8).
DNA molecules are made of two twisting, paired strands. Each strand is made of four chemical units, called nucleotide bases. The bases are adenine (A), thymine (T), guanine (G) and cytosine (C). Bases on opposite strands pair specifically; an A always pairs with a T, and a C always with a G. The human genome contains approximately 3 billion of these base pairs, which reside in the 23 pairs of chromosomes within the nucleus of all our cells. Each chromosome contains hundreds to thousands of genes, which carry the instructions for making proteins.