Are there any online tool for translating a Russian-language PDF to English?
During my PhD work, I've come across many non-English articles of interest that have no English translations best I could tell. Copying and pasting into Google Translate is good when you have a text or vectorized file. If you do not, OCR software, careful editing, and then copying and pasting into Google Translate works.

A member of my research group pointed out to me that you can get Google quality OCR by uploading the document to Google Drive and then editing in Google Docs. This can work very well, or not well at all. I typically find I need to clean up the scan enough in an image editing software that using a local OCR software is sufficient, so there's often no reason to use Google's OCR. I use Tesseract for local OCR. In your case, no OCR is required.

For longer documents, Google Translator Toolkit can be better than copying and pasting small segments. This will translate the entire document. I'd copy and paste the text of the document you are interested in into a text file and upload that to Google Translator Toolkit. Afterward, you can readd the equations and whatnot in Word, LaTeX, or whatever you believe would be best.

Also, my own experience with Russian and German in Google Translate for fluid dynamics related papers suggests that the translation often leaves something to be desired. Google Translate tends to give literal translations which may not correspond to what is said in English. (Frequently the differences in terminology are thought provoking, actually.) A scientific dictionary can help here, as also can having a copy of a related foreign language article and a professional translation of that article (to see how certain phrases are translated by a professional). All is not lost with Google alone, however. Google does take into account the edits you can make in Google Translate, and I've noticed over the past few years marked improvements that appear to come from my own edits.