I recommend honesty.... but backed up with tact. It worked for me. However, I was very lucky in my case because my partner already knew me in some capacity. So I think my revealing it actually made her go 'ahhh, well that makes sense'. Had it been a complete stranger it could have been a lil too much. So I certainly recommend honesty - but reveal it slowly maybe? Also, reveal it in a way that makes it seem 'just part of you' rather than something that is 'bad' about you....... because quite simply - its IS just part of you.
I guess what I mean is - reveal it as what it is. It's a normal reaction to abnormal circumstances. So you don't have to come forward with this great 'confession' that there is something wrong with you. There's actually nothing wrong with you. You're quite a normal person who has been through a lot. Revealed in the correct manner - that can actually come across as experience and wisdom.
Having said that - I never use my partner as a 'therapist'. She's not. If I am having a bad day I share this with her. But I don't 'offload the lot'. I may share that I am depressed and need to sleep. I don't spend 4 hours telling her why. If I am anxious I say 'I am feeling anxious' - again, I don't spend 4 hours telling her why. She is my partner, not my therapist. And so far ...*fingers crossed*.... this is working. I also allow her to 'offload' on me. But never to the point I am burdened. She may not have PTSD, but she has been through her own life trials. And she does have 'bad days' just like me. I try to make the 'lean on me' thing as equal as I can. And if it's ever too much, we simply say so.