The Bible didn't exist at the time Paul wrote his letters.
This is inaccurate, dear wannabefree (peace to you!). Much of the Tanakh (Jewish canon of Hebrew Bible) was completed before 400 BCE, and the Septuagint (or "LXX", the Koine Greek version of Hebrew Bible) was completed in Alexandria, Egypt, before 100 BCE. They were in existence in the time of my Lord's flesh... and were considered "Bibles" or, rather, the "Scriptures." They are, in fact, the "writings" (Greek, graphe) that my Lord referred to when he said, "Woe, to you... scribes!" As to both, actually... and what all other "versions" were out there at the time.
Did he believe that the letters he was writing were scripture?
I cannot speak to what Paul believed when he wrote his letters (that they are merely letters and not prophecies would make be believe he didn't believe them to be "scripture"); however, given Paul's numerous errors IN them... I would say that they weren't. Because the Holy Spirit doesn't tell you to judge one day... and then not to another. Paul was a former Pharisee and had to learn things... as he LEARNED to walk by spirit and not by sight. Didn't happen for him overnight.
Did he mean that as a general statement to encompass everything?
Problem is... didn't matter. For Paul to say, in one place, that "All scripture is inspired and beneficial for teaching, etc."... but "whenever Moses is read a veil lies on their hearts..." is telling. For him to say, "Do you not judge those on the inside...?" in one place then, later, "Why do you judge your brother? ... Therefore, let us not be judging one another any longer..." should tell us that he was not inspired by the Holy Spirit.
Would he have had something in particular in mind that he was referring to?
The scriptures, which Christ "opened up" to his disciples... and not they to themselves. That's why John had a "revelation." A revealing. Given to him, by Christ, through the arkangel, Michael.
For example, the book of Acts, would that be inspired scripture or simply historic account of the apostles from Luke's research? (I think Luke wrote that.)
The latter. How do we know? Because we know that Theophilus commissioned him, not God. And we know he got his information from interviewing witnesses... not God. Scripture, however, is what writers received from God... through His Word... which Word told them to write... and bore them along by the holy spirit as they did write. That's why THEY (the writers)... didn't need anyone to be teaching THEM: because the anointing THEY received... when they were "chosen" to write and DELIVER the message... taught THEM.
In 2 Peter 3:6, Peter refers to Paul's letters and talks about "the other scriptures," indicating that he considered Paul's letters to be scripture.
Smile! Here is another instance of man leaning upon his own understanding. The Greek word for "scriptures"... is the SAME as the Greek word for "writings." Since Bible copyists lean upon their OWN understanding... they very often miss which definition of a word should be used where. For example, the same Greek word that refers to my Lord as the Word (capitalized)... is used for the common every day word "word" (not capitalized). So, there the verse seems to indicate Peter referring to other "scriptures," he is not. He is referring to the other "writings"... of which there were MANY. As Luke wrote:
"Whereas many have undertaken to compile a statement of the facts that are given full credence among us..." Luke 1:1
Problem is... only four (4) were included in the Bible canon. Only ONE of them was written at/near/around the time Luke wrote his account. So, where are the other "many" accounts that Luke referred to? Probably in the back room of the back room of a back room, under the back room... of the Vatican, or Greek Orthodox Church, or some such place. Or maybe where Paul's FIRST letter to the Corinthians is reported to have gone: into the fire. Otherwise, they are out there, dear ones. These "writings." The "other" writings. But none of them are "scripture."
I bid you all peace!
A slave of Christ,
SA