If your image looks as it should when you zoom in there is nothing wrong with your image. OpenOffice may be resampling or compressing your image, but if it looks as it should at a higher zoom level then I assume that's not the problem. The problem is how Adobe Reader is rendering your image.
It's physically impossible for any image to render perfectly at any scale. Your image is made up of pixels, if you scale your image by any non-integer amount those pixels don't align with the new scale and you get those jagged edges (aliasing), which are especially noticeable on thin lines.
At a guess, the 140% zoom in Reader is probably showing your image at 100% (or some integer scale) in regards to the images actual pixel size and your display. This may be due to you scaling the image in OpenOffice or just due to Reader basing its scale on the physical dimensions of the PDF not the pixel dimensions of the image inside that PDF.
If this is destined for print you shouldn't need to worry about it, the issue is a screen display problem that won't translate to a printer.
One thing to check is Readers rending preferences to see if anything there is affecting the quality. (Preferences > Page Display > Rendering). I don't have Reader installed here so I'm not sure but I know there are font rendering options there.
Regarding your workflow, you can export a PDF directly from Photoshop so there's no need to use OpenOffice for that. A more usual workflow would be exporting a PDF or high res TIFF from Photoshop and using InDesign or some other page layout software to put everything together.