How do I import documents pertaining to a person after I started the family tree?

Answer 1

Unlike some other Family Tree applications (e.g. Ancestry), Family Echo doesn't support the importing of documents, other than a photo of an individual.

Depending on the content (text) and length, you could copy/paste the contents of the document into the Bio Notes field of the Biographical tab.  It holds quite a bit of text, though I don't know the limit.  The Interests and Activities fields also are designed to hold free-form text.

Answer 2

One thing you might consider is to use a free website service like Google Sites to create a simple web page displaying or linking to the the documents.  Then point to the web page in the Website field on the person's Contacts tab.

Here is a video about Google Sites.

Somewhat simpler would be to create a document with your favorite word processor that contains all the stuff you want to share.  Save it as a pdf file.  Upload the pdf file to a cloud service like Google Drive.  You can then link to that file as mentioned above.