A family of Microsoft word processing software products for creating web, email, and print documents.
As Stefan replied these most likely are Section Break (Next Page) which are inserted intentionally for a variety of layout changes as well as other purposes. It is virtually impossible to insert them accidentally.
They can be removed easily but doing so can have a detrimental effect on the flow & structure of the document because the properties associated with the Section will be removed as well.
To delete a Section Break place the insertion point at the beginning of the break line then either press Shift+→ , delete or press fn+delete if the kybd has a delete key.
Alternatively, place the insertion point at the end of the break line then use Command+← , delete.