I ran the following command
> pdfcpu merge out.pdf Lead-Based_Paint_and_Lead-Based_Paint_Hazards_Disclosure_1_-_1221_ts62956.pdf Pet_Addendum_-_1113_ts59831.pdf Res_LeaseMonth-to-Month_Rental_Agmt_-_1221_ts58268.pdf Addendum_No_1_-_1221_ts67645.pdf KeysafeLockbox_Addendum_and_Tenant_Permission_to_Access_Property_-_413_ts61394.pdf

But it shows me following error:

EmbeddedFilesNameTreeValue: unsupported in version 1.3
This file could be PDF/A compliant but pdfcpu only supports versions <= PDF V1.7