Why are my received file attachments (Office files such as .doc, .docx, .xlsx but also pdf files) corrupted - so that I cannot open them?

Question

For the last month, 90% of the file attachments that I receive by email will not open on my MacBook Pro (2009, running Mac OS 10.7.5). I get assorted messages, but generally saying that the file is corrupt, or the filename contains unacceptable characters. The problem has been generally with Word files (.doc and .docx)], Excel files (.xls and .xlsx) and pdf files.
The bit that I don't understand is that I can open the same email attachments (without any problem) when downloaded on my iMac. And I have recently found that colleagues who received the same emails can open them, and can transfer the files to me (usually by USB stick) … and then I can open them. But I still cannot open the same files received as email attachments on my MacBook Pro.
Which makes me think that the files (email attachments) are somehow being corrupted as they are downloaded?
I installed the Mac OS X Update Combined 10.7.5 in July, so perhaps this was when the problem started? And, although it may be unrelated, around the same time as this problem started, I transferred some files from a colleague's computer on a USB stick and found that the USB stick had picked up a virus/malware, which meant that the stick was dead, and I had to reformat the stick. I scanned my computer (using ClamXav, with the latest updates) and quarantined a file found on the USB stick and on my computer.
Any thoughts on why this might happen, and what I can do to sort it out (as it is making work very difficult - I travel a lot, and rely on being able to open and read files sent from countries all over the world, often by people using old versions of Office software)? I have never had problems in the past, and can't understand why my other computer, and my colleagues, can all open these files, but my laptop will not! Many thanks for any help!    

Answer 1

I opened one of the PDF files in notepad and these are the first couple lines:
瀖ᕁމጿ␠씴豈䧉筩롈ꓳ劏ꎯ僪뚢頟뻏즏谀㿃夑퀉꟠鲲쮂⫉笿褡밤籞冁탁ӓ轸뿐笼ⵆ횰䄌ඁ淥ة寨闤⬅鳦팥링빨嬄敎უ婏㣴ً鮓ࣿꢚ㑀녲莒඼ိȆ䕇纍쉉籶뺝갞伐쮠᥏﯒넉釖ȓ겴☧ ἣ秵駻�䣞띰㖔流羀籔朼敨ꢉ糶당⬤俉膇䄐惡�ಹꛖ鍡恡ⱶᶜ�堷﹑ﱌ僿걄뎔æ䋷귪⛢⫐䅪䉙὿烶ꖆ႟ᗔ瘞狻틫儩六잶覱낵듘盋崾�ᦜ㺆௹뻹燴ឋ騙쬄ꏿ뽒煹�钼뇲腎稦ꃲ㈃沒ꔈ뺐뛽첑䘶畱䍣紻 ⁜哠鳾