The only file type the FD Go and Mobile app can open is FDX, the format used by Final Draft. It does not open FDR (Final Draft 7 / 6 / 5), RTF, DOC, TXT, HTML or PDF files.
The Go and Mobile app cannot open text files created in Final Draft.
Some other screenwriting programs can save as FDX, but these files are not written in genuine Final Draft FDX and the scripts may not look or behave as expected when opened in Final Draft or Final Draft Go or Final Draft Mobile.