If you only download one, make it A Sanskrit-English Dictionary by Sir Monier Monier-Williams. —you asked for English-to-Sanskrit, correct? Most classic works are actually Sanskrit-to-English.
Sanskrit is not a dead language; it is a dormant one. Every word you look up in an is a resurrection. Whether you choose the exhaustive depth of Monier-Williams or the practical clarity of Apte, the act of searching bridges two worlds. english sanskrit dictionary pdf