“Reaches” is a verb and object combination that indicates the arrival at a particular stage or for having attained something. It is used in the context of quantities to refer to the mark it has achieved and else also used to indicate the end of a journey.
“Subscribers” is a word that refers to a person or group of people who have contracted for something by paying money or expressed their approval for someone. It is combination of a verb and an object and is used always in its simple form.
“Text-to-speech function” is a compound phrase or multi-word item that indicates a certain aspect of a device or role. It is a postmodern reference that relates to the functionality of converting text to speech in the latest devices and is quite commonly used in the noun form itself.
“Digital books” is a multi-word that has reference to the non-physical form of a reading material and is a noun that is used formally in texts.
“eBook Reading Devices” is a compound phrase that is collectively termed to a class of gadgets that are used for the electronic reading of books. It is a noun that is used in the verbatim sense with a reference to reading.