Translation:The lawyer spoke about the disappearance of documents.
That would be "dos" (de + os).
Using "de documentos" is about some documents disappearing here and there (it's a general statement)
Using "dos documentos" is about those specific documents that disappeared.
2019-09-05 The male voice pronounces advogado as "adji-vogado". I had read in a lower-level discussion that D and T were only fricative when in the last syllable of a word (including one-syllable words like de). Is that pronunciation correct? Is it a regional accent?
There are two phonological processed going on in this example. The first is that Portuguese-speakers don't like to end a syllable with a consonant sound or to have two consonants together like the 'd' and the 'v' in 'advogado.' Generally they will insert an 'i' (/ee/ sound in English) in between the two consonants. For example 'obter' is pronounced 'o-bee-ter.'
Now the second thing that happens is a 'd' or a 't' followed by an 'ee' sound will often become palatized to /ʤ/ (English 'j') and /ʧ/ (English 'ch') sounds, respectfully. Technically these two sounds are not classified as fricatives but are known as affricates.