HI Freek,
"It should be the real data of the document" - Yes, it should be binary data of the document
"The Rest API it selfs converts the text of the post to binary so in your case it will be converted 2 times." - Partially true, but not all bytes of a pdf are valid utf-8 characters, so unless your pdf is 100% ascii (which is hard because the header usually contains non-ascii charecters) you will lose characters in that first conversion.
"It should be the real data of the document" - Yes, it should be binary data of the document
"The Rest API it selfs converts the text of the post to binary so in your case it will be converted 2 times." - Partially true, but not all bytes of a pdf are valid utf-8 characters, so unless your pdf is 100% ascii (which is hard because the header usually contains non-ascii charecters) you will lose characters in that first conversion.