Proprietary Software can include back doors – see Skype and Microsoft.
Proprietary formats can include metadata. This is data, which you can’t see but it can lead to your identity. They caught a Greek anonymous activist, because he uploaded a word document with his real name in the metadata.
If you are no computer expert don’t upload anything else then plain TXT files to the Internet. You can use copy and past as well to post it in web services. Even graphic formats like JPEG or TIFF can include data like GPS coordinates, the used camera, user and software name.
It’s very difficult for beginners to find this metadata. So if you are a good designer like the poor Greek one, send your PDF files to a computer expert. He can clean the metadata before the upload.
These programms can show you the metadata:
The metadata can be useful to locate the author of a document in real life, if you have questions for example. Open source programs like Libre Office uses metadata too. The trick is not to fill in your real name during installation and don’t use your real name for login.