Caroline, as Donald points out, you have to select the tab “archivos” first – then use the tick box to checklist one or more designated archives
Then use pesquisa - search - whilst one the archive selction page.
Type into the box and click on the image immediately to its right
You will also no doubt quickly find that the search facility is quite sophisticated despite it being called "pesquisa simples".
For example you can type in “Freire” and get some 1200 responses or “Freire de Andrade” 162 responses
The search facility also appears to cover both the sender and recipient of correspondence
Best wishes
Ant